Rotary low-frequency sound reproducing apparatus and method

ABSTRACT

Apparatus (21) is described for reproducing low frequency sound in response to an applied electrical signal having a rotary acoustic radiator (22) comprised of a chamber (26) having a rotor assembly (58) including a shaft (61) and movable vanes (71, 72) secured to the shaft rotatably mounted in chamber for rotation on a longitudinal axis. Stationary vanes (62, 63) are mounted in the chamber and extend generally radially between the chamber walls and the shaft. The chamber has ports (64, 66, 67, 68) therein opening into the chamber and disposed adjacent the stationary vanes on opposite sides of said stationary vanes. The movable vanes are disposed on opposite sides of the stationary vanes and extend generally radially between the chamber walls and the shaft.

The invention relates to a rotary low frequency sound reproducingapparatus and method and more particularly to a computer-aided rotaryelectromechanical transducer and method for reproduction of sound in thefirst two octaves of the audible frequency spectrum.

BACKGROUND OF THE INVENTION

The reproduction of the first two octaves of audible low-frequencysound, from 20 to 80 Hz, is a long-standing problem in that largevolumes of air must be moved. For a given loudness of a reproducedlow-frequency sound, the air volume moved must be doubled for eachhalving of reproduced sound frequency.

Many solutions have been attempted. Low-frequency voice-coil-and-coneloudspeakers with highly compliant suspensions have been used. However,the long cone travels needed are difficult to achieve with linearity.Nonlinearities introduce intermodulation distortion, the modulation ofhigher reproduced frequencies by lower reproduced frequencies. Moreserious attempts to produce the lowest frequencies in sound isolate thelower two octaves from higher frequencies for reproduction through"subwoofer" loudspeakers.

Direct radiator voice-coil-and-cone loudspeakers, which have at best anefficiency of a few percent, have been used as subwoofers in 12 to 30inch frame sizes in large cabinets of several cubic feet in volume.Required driving powers are also large, typically several hundred watts.Subwoofer low-frequency response is limited by resonance of thevoice-coil-and-cone masses in combination with compliance of thevoice-coil support "spider", the cone flexible surround, and the aircompliance of a closed cabinet, if used. Together these set an effectivelower limit to the frequencies of output sound since it is difficult todrive a loudspeaker below its low-frequency resonance. Closed-loop(negative feedback) servomechanisms controlling voice-coil movementshave been used to reduce these effects but they may leave the cone andflexible surround free to flex in unwanted modes at high amplitude,limiting usable power output by the onset of distortion.

In auditoria and stadia, horn-type radiators driven by fairlyconventional voice-coil-and-cone transducers have also been used assubwoofers with efficiencies in the 30 to 50 percent range. Theirlimitation in the domestic environment is that for reasonableperformance the perimeter of the horn mouth must be of the order of thewavelength of the lowest reproduced sound, e.g., over 50 20 feet at 20Hz. Recently, horns of the type disclosed in U.S. Pat. No. 4,564,727have been driven from externally cooled servomotors through pulley,belt, and cone arrangements, achieving remarkable sustained acousticoutputs.

Most recently, in U.S. Pat. No. 4,763,358, the use of apositive-displacement rotary-vane pump is disclosed. If of suitable sizeit should be able to produce usable output to and below the 20 Hz limitof audibility. The rotary vane pump may be used to drive a horn, thoughthe required horn mouth size (above) practically excludes it from thedomestic user environment. If used as a direct radiator, rotary vanepump efficiency is similar to that of voice-coil-and-cone directradiators. At high output, most of the input power must be dissipated asheat, usually from within a closed cabinet. Though their volumetricefficiency is high relative to that of voice-coil-and-cone loudspeakers,since much of rotary vane device volume may be swept by the vanes, thedevices and their cabinetry can be large. Cabinetry structure fordevices having only full-length ports in pump-enclosure sidewalls isawkward. Motor wear and noise, bearing noise, and seal-leakage noise canbe problematic in a quiet, e.g., home or auto, user environment.Port-turbulence noise must be managed--a nominal 15 inch diameter, 9inch long rotary acoustic radiator moves about 6 times the air volume ina single stroke as does a conventional 15 inch loudspeaker.

Position sensing has heretofore been disclosed for rotary acousticradiators to provide negative feedback information active in the samefrequency band as the acoustic output and linearize vane travel. Thisapproach has been used successfully in voice-coil-and-cone loudspeakers,which are linear at null or neutral position. Rotary acoustictransducers are not linear at null. The support bearings of rotarydevices have static friction differing severalfold from dynamicfriction, and both static and dynamic friction vary with temperature andtime. Dynamic friction, determined more by grease seals than by thebearings themselves, increases with rotational velocity.

Bearing, slip ring, and motor-brush static friction induce distortion atlow output amplitudes. Total breakaway (from stop) torques, which aretypically 2 percent of full motor torque, are 20 percent of torque whenthe audio output level is down 20 db, and further increase the relativedistortion level with decreasing output. Since the usual dynamic rangeof entertainment audio is 40 to 50 db, such distortions at mid and lowamplitudes are serious problems. Commutation discontinuities andirregularities of motor magnetic fields also contribute somewhatunpredictably to low-amplitude output distortion, as their magnitude isoften a discontinuous function of motor armature rotational position.

Negative feedback adequate to contain these nonlinearities to auser-acceptable level would likely be 14 db or more, implying acorollary unity-gain negative feedback loop crossover in the region of300 Hz or higher. Stability is difficult to assure over a subwoofer'slife with such nonlinear electromechanical components and high bandwidthin a negative feedback loop.

Commutated motors, when used to drive acoustic transducers, introduce aspecial problem. A wide variety of techniques have been employed toreduce the characteristic of commutated motors commonly referred to astorque cogging or torque ripple, hereinafter referred to as torqueripple, which is the principal distortion-generating limitation ofcommutated motors when employed in rotary acoustic transducers. Theseripple effects occur when windings connected to rotationally adjacentcommutator sectors are shunted together by brushes. Brushless motorshaving multiple permanent magnet rotors and multiphase stator windings,particularly those having precision angular position informationavailable for use in commutation such as computer memory disk drives andthe rotary acoustic transducer of this invention, can be commutatedwithout the positional uncertainty and torque ripple arising through useof mechanical brushes. With electronic commutation angular gaps may beintroduced between stator connections during commutation to minimizeinductive and ferromagnetic hysteresis effects in stator windings duringphase connection and disconnection, as in Janssen U.S. Pat. No. 4703236.Separate windings for each pole set in a multiphase motor which share asingle driving source, such as a power amplifier, can limit theelectromagnetic disturbance during an event of commutation to one or twopole sets, rather than disturbing the entire stator during eachcommutation as in conventional lap or wave stator winding patternswherein all stator poles share the same winding circuit.

Torque ripple is reduced by the accurate commutation describedhereinabove. Nevertheless in brushless motors stator winding commutationgenerally occurs adjacent to rotor pole edges and is a source of torqueripple as stator pole magnetic flux reverses and stator pole fluxtransfers from one rotor pole to the next. Additional sources of torqueripple are cogging of the rotor from pole to pole of the stator due touneven flux distribution across pole faces and winding slots, fluxvariations across the faces of stator poles themselves, and polesaturation. These sources of torque ripple have been compensated byusing large numbers of stator poles; using numbers of rotor and statorpoles which are not multiples or submultiples of each other; skewing thestator poles from their usual radial or axial alignments in axial-gapand radial-gap motors, respectively; shaping the stator pole faces, aswith surface depressions, to produce a desired flux distribution, as inHertrich, U.S. Pat. No. 4,874,975; and modulating the stator windingdrive current with a repetitive pattern in synchrony with the multipolerotor assembly angular rotation over stator poles, as in Gotoh et al.,U.S. Pat. No. 4,525,657.

There is therefore a need to address these problems of low-frequencysound transducers, and in particular rotary-vane transducers, to producea low-frequency sound reproducing apparatus and method more suitable forthe consumer environment.

THE DRAWINGS

FIG. 1 is a partially exploded, isometric view of a loudspeaker cabinetincorporating the rotary acoustic transducer apparatus incorporating thepresent invention. FIG. 2a is a schematic isometric exploded view of therotary acoustical transducer apparatus with certain portions broken awayand an overview of its control system.

FIG. 2b is a block diagram of the rotary acoustic transducer apparatuscontrol system.

FIGS. 3 through 8 are exploded isometric views of the rotary acousticradiator assembly with certain portions being broken away.

FIG. 9 is an exploded view, with certain portions broken away, of a morespecific embodiment of a rotary acoustic transducer apparatusincorporating the present invention.

FIG. 10 is an isometric view of the armature shown in FIG. 9.

FIG. 11 is a partially exploded, isometric view of an alternativeloudspeaker cabinet and diffuser-attenuator incorporating certainaspects of the rotary acoustic transducer apparatus of the presentinvention.

FIG. 12 is a cross sectional view taken along the line 12--12 of FIG. 11but in an unexploded condition.

FIG. 13 is a computer-aided wireframe isometric partially exploded viewof an axial-gap multipole brushless commutated torque motor illustratingthe relationship of rotor pole segments and stator poles.

FIG. 14 is a schematic diagram of the winding pattern of an axial-gapmultipole brushless electric motor and its associated drivingelectronics.

FIG. 15a is a schematic representation of stator winding driving currentcommutation transitions of a typical electronically commutated brushlesselectric motor.

FIG. 15b is a schematic representation of stator winding driving currentcommutation transitions of an electronically commutated brushlesselectric motor of the present invention in which hysteresis has beenintroduced to said commutation transitions.

FIG. 16 is a computer-aided wireframe isometric partially exploded viewof rotary acoustic radiator having a generally spherical chamber withflattened axial ends.

FIG. 17 is a computer-aided wireframe isometric partially exploded viewof rotary acoustic radiator having a toroidal chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The rotary acoustic transducer apparatus of the present invention isused for producing low frequency sound in response to an applied audiosignal. It is comprised of a rotary acoustic radiator assembly, a torquemotor, a position sensor and a microcomputer. The apparatus may also beprovided with a supporting cabinet. The radiator assembly comprises agenerally cylindrical means with a cylindrical side wall and end wallsforming a cylindrical chamber having an axis. A shaft is provided.Bearings mount the shaft in the cylindrical means for rotation aboutsaid axis. A cylindrical hub is secured to the shaft and extends betweenthe end walls. Movable vanes are secured to the shaft. The shaft and huband movable vanes form a rotor assembly. Stationary vanes are mounted inthe chamber between said moving vanes and extend between the cylindricalside wall and the hub and between the end walls. The cylindrical meanshas ports opening through the cylindrical chamber walls to permit airflow into and out of the cylindrical chamber in response to movement ofthe movable vanes. A torque motor is coupled to the shaft for applyingrotational reciprocating movement to the movable vanes. A positionsensor, which may use direct rotational position sensing or indirectrotational position sensing by integration of velocity or acceleration,ascertains the position of said rotor assembly. Typically an incrementalposition sensor would be used to provide high resolution at low cost,but alternatively absolute position sensors may be used -- they do notrequire a separate indexing line or procedure as described hereinbelow.A microcomputer is provided and is coupled to the torque motor and tothe position sensor and controls the operation of the torque motor inaccordance with the applied audio signal. Nonlinearities arising in thetorque motor, bearings, seals, and cabinet are measured by the positionsensor and microcomputer and correcting signals created to modify theapplied audio signal. Displaced air may be used as a torque motorcoolant. A diffuser-attenuator slows user-side airflows and permitssmall user-side ports to be used, increasing volumetric efficiency. Thediffuser-attenuator is fabricated partially of acoustically-absorbentmaterials to attenuate vane-edge leakage noise and port-turbulencenoise.

More particularly, as shown in FIG. 1 of the drawings, the rotarylow-frequency sound producing apparatus 21 which alternatively can becalled a rotary acoustic transducer apparatus is comprised of rotaryacoustic radiator assembly 22, a torque motor 23 and a position sensor24, said apparatus typically being supported by a cabinet 26. A typicalcabinet 26 is provided with four side walls 27, a top wall 28 and abottom wall 29 to form a rectangular cabinet or enclosure. The top wall28 is provided with ports 31 and 32. The rotary acoustic radiatorassembly 22 is secured to the wall 28 by suitable means, such as bolts(not shown) so that the ports 31 and 32 in the cabinet top wall 28register with the ports hereinafter described in the rotary acousticradiator assembly 22. Cabinet minimum volume is in the range of three toten times the maximum air volume which may be displaced by the rotaryacoustic radiator assembly 22 in a single stroke of the rotary acousticradiator assembly 22.

A diffuser-attenuator 36 is comprised of a top planar baffle 37overlying and spaced upwardly from the cabinet top wall 28. The topbaffle 37 has approximately the same area as the top wall of the cabinet26, and is spaced upwardly from the cabinet top wall 28 a suitabledistance, as for example one fourth of the diameter of the rotaryacoustic radiator assembly diameter 22. Fixed air duct baffle means 38,39, and 41 are provided between the top baffle 37 and the cabinet topwall 28. The top baffle 37 is supported on the cabinet top wall 28 byspacer and bolt assemblies (not shown) placed as appropriate to supportthe top baffle 37 and inhibit vibration of the top baffle 37. Air fromthe ports 31 and 32 enters the diffuser-attenuator 36 through slots 42and 43 and is redirected so that it exits in the four mouths 44 in thediffuser-attenuator between the top baffle 37 and cabinet top wall 28.Air exiting from the ports 31 and 32 of the cabinet 26 can haveconsiderable kinetic energy which, as the air passes through thediffuser-attenuator 36, is converted to potential energy in a sound waveby reducing air velocity. Air entering the ports 31 and 32 is driven bythe potential energy of ambient air pressure and is shaped into arapidly-moving air column. Though nomenclature is similar, no attempt ismade to make the diffuser-attenuator 36 function as a traditionalacoustic horn radiator.

FIG. 2a is a schematic illustration of the low-frequency soundreproducing apparatus 21 without a cabinet 26. As shown, the rotaryacoustic radiator assembly 22 consists of a right circular cylinder orenclosure 51 formed by a cylindrical sidewall 52 which has alongitudinal axis 53. First and second or top and bottom spaced apartparallel circular end walls 54 and 56 adjoin the cylindrical sidewall 52to form a closed cylindrical chamber 57. A rotor assembly 58 is mountedin the right circular cylindrical chamber 57 and is comprised of acylindrical hub 59 which is adapted to rotate on the longitudinal axis53 coaxial with the cylindrical sidewall 52 by a shaft 61. Thecylindrical hub 59 extends between the first and second end walls 54 and56 and is rotatably carried by the shaft 61. The shaft 61 extendscoaxially of the hub 59 and is rotatably mounted in the end walls 54 and56 by suitable anti-friction bearings (not shown). In the embodiment ofthe invention shown in FIG. 2, the shaft 61 extends through the secondend wall 56 for a purpose hereinafter described. It should beappreciated that if desired, the shaft 61 can be formed in two parts andneed not necessarily extend through the hub 59. For example, the twoparts may be secured to opposite ends of the hub 59.

First and second stationary vanes 62 and 63 (see FIGS. 2 and 3) aremounted within the chamber 57 of the right circular cylinder means 51and extend radially of the chamber 57 between the cylindrical sidewall52 and the hub 59 and between the first and second end walls 54 and 56.Thus, as shown the stationary vanes 62 and 63 can be secured to thecylindrical sidewall 52 and the first and second end walls 54 and 56 andcan extend into very close proximity to the hub 59 but not touching sameso as to frictionally engage the hub and impede rotational movement ofthe rotor assembly 58.

First and second ports 64 and 66 in the form of radial sectors areprovided in the first end wall 54 and are disposed counterclockwiseadjacent to the stationary vanes 62 and 63. Thus, as shown in FIGS. 2and 3, a port 64 is provided in end wall 54 adjacent to stationary vane62 and a port 66 is provided in the end wall 54 adjacent to thestationary vane 63. In a similar manner ports 67 and 68 in the form ofradial sectors are disposed in the second end wall 56 clockwise adjacentto stationary vanes 62 and 63, respectively. The ports 64, 66, 67 and 68may have a wide variety of configurations, as described hereinafter.

The rotor assembly 58, in addition to the hub 59 and the shaft 61includes first and second movable vanes 71 and 72 which are secured tothe hub 59 and extend radially therefrom into very close proximity tothe cylindrical sidewall 52. They also extend between the first andsecond end walls 54 and 56 but do not frictionally engage them. Themovable vanes 71 and 72 may be tapered as shown to provide greaterthickness and strength at the roots or proximal extremities of the vanesto resist motor torque while being narrower at the tips or distalextremities to provide low mass and hence low rotational moment ofinertia. The movable vanes 71 and 72, which are subject to largeaccelerating torques from the torque motor 23 and large air pressureforces generated within a cabinet 26, must necessarily have a rigidstructure. These rigid structures are not a source of harmonicdistortion as are the more flexible cone and cone-surround components ofa conventional loudspeaker.

The large hub 59 reduces the angle about the longitudinal axis 53subtended by the thickness of the inner or axial margins of thestationary vanes 62 and 63 while containing the robust structureincluding the hub 59 securing the movable vanes 71 and 72 to the shaft61. Most of the hub included volume may be structural foamed plasticwhich contributes little to the rotational moment of inertia of therotor assembly 58.

The right circular cylinder 51 and the stationary vanes 62 and 63 andmovable vanes 71 and 72 may be formed of a number of suitable materialssuch as metal or composites provided they are dimensionally stable andhave closely matched thermal expansion characteristics. Use of therotary acoustic radiator assembly 22 as a heat sink for the torque motor23 favors the use of high-thermal-conductivity metal such as aluminumfor these components.

The control functions of the rotary acoustic radiator assembly 21 arediscussed in some detail hereinbelow; an overview is shown in FIG. 2a.The torque motor 23 serves as means for applying rotationalreciprocating movement to the rotor assembly 58 through the shaft 61.The drive signal to the torque motor 23 is provided by a power amplifier75 through lines 76 and 77 with a ground return line 78 connected toground through a low resistance armature-current sampling resistor 79. Adriving signal for the power amplifier 75 is provided on a line 80 froma microcomputer 81 which has as its primary input a low-frequency audiosignal applied on line 82. The position sensor 24 provides positioninformation of the rotor assembly 58 to the microcomputer 81 throughlines 83 and 84. Another line 85 from the sensor 24 to the microcomputer81 serves as a ground line. Armature current information is provided tothe microcomputer 81 on a line 78 in the form of the voltage drop acrossresistor 79. Line 87 to the microcomputer 81 samples the drive voltageapplied to the torque motor 23.

In FIG. 2a the microcomputer 81 includes analog to digital (A/D) anddigital to analog (D/A) converters 88 for converting signals betweenanalog and digital forms as appropriate for processing in themicrocomputer 81 and other electronic components of the rotarytransducer assembly 21. The microcomputer 81 also includes a delaybuffer 89 for management of distortion, as described hereinafter.Alternatively A/D, D/A, delay buffer, rotor assembly 58 position,angular velocity, and angular acceleration functions may be determinedwith circuitry external to the microcomputer 81, as for example anapplication-specific integrated circuit (ASIC). This configuration isshown in the more detailed drawing of the control system FIG. 2b.

Operation of the rotary transducer assembly 21 shown in FIGS. 2a and 3may now be described. As the shaft 61 is rotated counterclockwise by thetorque motor 23, the hub 59 and the movable vanes 71 and 72 securedthereto are moved to cause air to be drawn into (inducted into) theports 64 and 66 at the first end wall 54 of the right circular cylinder51 and air forced out of (exhausted from) the ports 67 and 68 on thesecond end wall 56 of the right circular cylinder 51. When rotation ofthe hub 59 and the movable vanes 71 and 72 is reversed, air is exhaustedfrom the ports 64 and 66 and inducted into ports 67 and 68. Air leakagebetween the movable vanes 71 and 72 and the cylinder sidewall 52 and theend walls 54 and 56 and between the stationary vanes 62 and 63 and thehub 59 is limited by providing very close fits between the same,typically one one-thousandth of the internal diameter of the cylindersidewall 52.

As described hereinabove, a rotary acoustic transducer assembly 21includes a number of distortion sources which must be corrected orcompensated to produce a user-satisfactory apparatus. These correctionsare first derived in a startup protocol; some are updated duringsubsequent operation of the apparatus.

When power is first applied the microcomputer 81, in conjunction withthe A/D and D/A converters 88, the power amplifier 75, the signal delaybuffer 89, the position sensor 24 and the armature current-sensingresistor 79, performs a number of initializing functions in a startupprotocol comprised of rotor assembly 58 centering, torque motor 23linearity calibration, damage-protection braking table generation,cabinet 26 volume measurement, rotor assembly 58 air leakagemeasurement, bearing friction measurement, and armature resistancemeasurement. Following the startup protocol the applied audio signalV_(in) 82 is operated on by the microcomputer 81 to produce minimaldistortion in the reproduced sound, and other signals are originated asnecessary within the microcomputer 81 to manage and protect the rotarytransducer assembly 21. In the present invention, the techniques usedwhen reproducing sound, with the exception of centering as describedhereinbelow, are considered to be forward corrections, estimations, orpredictions based on accumulated historical data, as distinguished fromnegative feedback; during the startup protocol several processes utilizenegative feedback of limited bandwidth--under 10 Hz.

Execution of the startup protocol can be lengthy, particularly if thefit of rotor assembly 58 to cylindrical enclosure 51 is close,minimizing air leakage, and the cabinet 26 is not vented to abbreviatepressure equalization. Since data collected during the startup protocolis reasonably stable and may be stored in computer memory, startupfunctions may be exercised selectively; intervals between executions ofspecific startup protocol functions may be scheduled or executed oncommand.

FIG. 2b is a block diagram of the control system of FIG. 2a. Note thatseveral of line numbers of FIG. 2a are duplicated in FIG. 2b (80, 82,87) and suffixed with A or D, in each case indicating respectivelyanalogue (A) or digital (D) form of the same signal. Digital dataresolution is typically 14 or more bits. As noted above, in this FIG. 2bthe A/D-D/A 88, and delay buffer 89 functions have been separated fromthe microcomputer per se for clarity of presentation and also as anillustration of a desirable implementation form in which data arecollected, converted, and transmitted, most conveniently by directmemory access, to the microcomputer 81, at a typical data sample rate of3000 samples/sec as determined by the clock 90. Delayed data from thedelay buffer 89, use of which is described hereinbelow, enters themicrocomputer through line 91.

FIG. 2b is in a block diagram form common to description of traditionalanalog control systems, though the implementation is largely digital.This form facilitates a simple function-by-function narrativedescription of a fairly complex control system. The arguments of time,frequency, period, or applicable transforms, of which there are several,and array subscripts, are not included in the FIG. 2b annotation in theinterests of simplicity. The rotary acoustic radiator 22 and cabinet 26are not shown in this FIG. 2b as they are well illustrated in severalother Figures.

Fn is a function generator used in the startup protocol or inoperational device management functions producing voltage-equivalentdata signals entering the microcomputer 81 on line 92. G, enclosed in adashed line, is the active control element in the controlled system,including the microcomputer 81, the power amplifier 75, the motor 23,and the position encoder 24. H_(n) is a feedback or historical datareturn function. H_(n) voltage-equivalent data signals are returned tothe microcomputer on line 93. A primed function, e.g., H₂ ' is arepresentation of computer-manipulated forward correction or estimationdata derived from historical data stored in corresponding unprimed datareturn function, e.g., H₂ in microcomputer 81 memory. Correctivevoltage-equivalent data signals, e.g., V_(t1), are selected from theprimed H_(n) ' tables. Switches Sw_(n), are in common notationsubscripted, where appropriate, to correspond to their respective F_(n)functions. Note that with the exception of H₁ and H_(1A) (with S_(1c) orS_(1d) closed, respectively) no direct negative feedback connection isshown.

All data return functions H_(n) are shown for convenience as drawingdata from a common bus 94, the specific data elements used in the H_(n)function being shown adjacent to the line joining the right of eachH_(n) function to the data bus 94. Processed forward correction datafrom the H_(n) ' functions is indicated to the left of each H_(n) 'function.

Though the microcomputer 81 is shown as a separate element, it is to beunderstood that F_(n), V_(n), G, H_(n), and Sw_(n) functions are inpractice included in the microcomputer 81 and executed by it.

In the following table the variables used in the control system aredefined. The subsequent narrative describes, referring to FIG. 2b, thedevice initialization and operational control functions. Accompanyingeach such description is an example pseudocode computer program segmentwritten in language like a contemporary enriched BASIC illustratingimplementation of the control function algorithms. No attempt is made toreflect algorithm changes required by the interruptions and interactionsthat would obtain in an operating apparatus.

Control constants and variables. In the discussion of the preferredimplementation hereinbelow the following constants and variables areknown or derived as described from the rotary transducer assembly 21described above.

C, the index or centered, neutral position of the rotor assembly 58, asset by rotational positioning of the position encoder 24 duringapparatus fabrication.

S, rotor assembly 58 angular state relative to the index position C,clockwise (cw) or counterclockwise (ccw), indicated as a binary value.

Δt, the sampling period, typically 0.33 milliseconds, or a rate of 3000samples/sec.

t_(B), is bearing non-rotation or stop time.

t_(W) is a waiting period

.O slashed., rotor assembly 58 angular position, measured from the indexposition C, typically calculated every sampling period Δt, with aresolution of ±10,000 counts. Positive values of--are ccw.

d.O slashed./dt, rotor assembly 58 angular velocity, typically derivedas first differences of .O slashed., calculated every sampling periodΔt.

d² .O slashed./dt² rotor assembly 58 angular acceleration, typicallyderived as second differences of .O slashed., calculated every samplingperiod Δt.

Δ.O slashed., rotor assembly 58 angular position step value, typically 5or fewer degrees, used in the startup protocol to calibrate the rotarytransducer assembly 21 and as the ordinate increment of thelinearization table in microcomputer 81 memory, described hereinbelow.

±.O slashed.max, are limit rotor positions, measured in either directionfrom the index position C, typically 60 degrees.

.O slashed._(s) is the angular rotation prior to a stop.

L_(a), armature inductance, typically 3 millihenrys

R_(a), torque motor 23 armature resistance, determined during apparatusfabrication or by microcomputer 81 driven test, typically 2 ohms.

R_(a), armature current sampling resistor 79 resistance, determinedduring apparatus fabrication, typically 0.1 ohm.

V_(in), the applied audio signal on line 82, typically 1 volt RMSmaximum in analogue form.

V_(s), the voltage across sampling resistor 79 R_(s), measured on line78

V_(d), motor drive voltage, measured on line 77

V_(a), motor armature voltage, =V_(d) -V_(s).

I_(a), motor armature current, =V_(s) /R_(s)

V_(a) is a corrective voltage derived from historical data.

Control constants and variables (cont).

J, rotor assembly 58 moment of inertia (includes torque motor 23armature moment of inertia), determined during apparatus fabrication,typically 0.0004 slug-ft².

T, torque motor 23 torque, typically 20 ft-lb maximum.

A, power amplifier voltage gain, typically 50.

r, radius to centroid of movable vanes 71 and 72, measured from thechamber axis 53.

A_(v), Area of movable vanes projected on plane having chamber axis 53as one edge

dT/dI_(a), torque motor 23 armature current sensitivity, a function of φand Ia as the motor approaches saturation.

P, cabinet 26 internal pressure, also acting on rotor assembly 58.

V, cabinet 26 internal volume (includes connected volume of rotaryacoustic radiator 22)

Centering--initialization. The position encoder 24 provides a rotorassembly 58 binary indexing signal on line 83 and position informationon line 84. The indexing signal on line 83 indicates that the rotorassembly 58 is on one side or the other of the index (neutral, centered)position C. These indexing data and position data are periodicallysampled by the microcomputer 81 at the nominal rate of 3000 samples persecond.

During the centering function of the startup protocol the applied audiosignal V_(in) on line 82A, digitized in the A/D-DA converter 88 andreturned to the microcomputer 81 on line 82D is suppressed by openingSw₀. Sw_(1c) and Sw_(1d) are open. Sw_(1a) is closed. The binaryposition S, cw or ccw, of the rotor assembly 58 relative to the indexposition C is determined by the value of the binary signal on line 83and S on the digital data bus 94. A small voltage-equivalent signal v₁is generated in the microcomputer 81 by F_(1a) and Sw_(1b) is closed.This small voltage exiting the microcomputer 81 on line 80D is convertedto analogue in the A/D-D/A converter 88 and applied via line 80A to thepower amplifier 75 having voltage gain A to rotate the rotor assembly 58toward the index position C. When the binary value on lines 83 and 94change state, angular-position-recording (.O slashed.) registers inmicrocomputer 81 or external support circuitry are initialized andthereafter store the angular position .O slashed. of the rotor assembly58 measured from the index position C, as measured in the immediatelypreceding sampling period. Sw_(1a) is opened, disabling indexing statesampling S, SW_(1b) is opened, disabling F₁ and Sw_(1c) is closed,activating a low-bandwidth position feedback centering loop H₁.

Thereafter position data from position encoder 24 are returned to themicrocomputer 81 via the A/D-D/A converter 88 and the digital data bus94 and processed with appropriate gain adjustment and digital filteringin HI so that a negative feedback loop is created having a unity gain at0.5 Hz with a slope of -20 db/decade. At about 2 Hz the gain slope isincreased to -40 db/decade so that negative feedback and its attendantreduction of output is negligible in the audible range of 20 Hz andabove. The centering process may be suspended by opening Sw_(1c), haveits bandwidth increased by closing Sw1d to invoke H_(1A), or have itscenter position modulated by other apparatus management functions, asdescribed hereinbelow.

H_(1A), activated by opening Sw_(1c) and closing SW_(1d), is a feedbackpath having greater bandwidth used to abbreviate position settlingduring the startup protocol and for transient fast centering duringhigh-output operation as described hereinbelow. Typical unity gain is atabout 8 Hz with a local gain slope of -20 db/decade and gain slopeincreased to -40 db/decade at about 30 Hz. Typically this fast feedbackloop from data bus 94 through H_(1A) must damp the rotary-acoustictransducer 21 and cabinet 26 resonance, which occurs in the 3-6 Hzregion, below loop unity-gain crossover.

Centering--operation. In normal operation Sw₀ and Sw_(1C) are closed,applied audio signal Vin on line 82A is digitized and amplified anddelivered to the torque motor 23 as described above while thelow-bandwidth centering feedback system through H₁ and Sw_(1C) continuesto operate with negligible effect on reproduced sound.

Example pseudocode centering algorithms are: ##EQU1##

Torque motor linearization--initialization. It is not economicallyconvenient to produce an uncommutated torque motor 23 with lineararmature deflections of the 1 to 2 radians needed for a rotary acousticradiator assembly 21. Uncommutated torque motor 23torque-to-armature-current sensitivity dT/dI, usually decreases at largeangular deflections .O slashed. and armature currents I_(a) and is apotential source of predominantly odd-order harmonic distortion. Thesenonlinearities must be measured and corrected.

During the startup protocol applied audio signal on line 82D issuppressed by opening Sw₀ and the torque motor 23 is stepped through itsoperational range of rotation .O slashed. in increments of a few degreesΔ.O slashed. by a sequence of corresponding voltages V_(2a) originatedat F_(2a) in the microcomputer 81. SW_(1d) is open, disabling fastfeedback. SW_(1c) is closed so negative feedback is used to assurecorrect angular positioning of the rotor assembly 58 at each angularstep. The rotor assembly 58 is retained at the desired angular stepposition .O slashed. by the feedback until the torque T required toposition it is substantially zero, as indicated by armature currentI_(a) =0, measured as V_(a) =0 on line 78. In general the required timeto achieve zero torque T is established by air leakage around the rotorassembly 58 which extinguishes the pressure difference between cabinetand user sides of the rotary acoustic transducer 22. Typically severalseconds are needed for this pressure-difference decay. Anelectrically-operated vent controlled by microcomputer 81 via Sw₈,venting the cabinet 26 interior to outside air, may be used toaccelerate the pressure-difference decay.

At each rotor assembly 58 angular step .O slashed., when the pressuredifference has decayed (I_(a) or V_(s) =0), the signal v_(2a) fromF_(2a) is suppressed by opening SW_(2a), the position negative feedbackis suppressed (Sw_(1c), Sw_(1d) open) by the microcomputer 81, SW_(2b)is closed and a voltage pulse v_(2b) from F_(2b) is converted to a knownpulse of armature current I_(a) by the power amplifier 75 and applied tothe armature of the torque motor 23. This pulse produces an angularacceleration d² .O slashed./dt² of the armature and rotor assembly 58,which is detected by the position encoder 24 and microcomputer 81 as aninitial movement, and is a measure of torque motor 23torque-to-armature-current dT/dI_(a) sensitivity at the measured angularposition .O slashed. and armature current I_(a). The armature currentpulses are brief --about 2 milliseconds--so that resulting rotorassembly 58 angular deflection is small and significant cabinet 26pressure differences do not develop across the rotor assembly 58 duringthe pulse. With large armature current I_(a) pulses, small pressuredifferences do indeed develop across the rotor assembly 58 due to rotorassembly 58 rotation, and small torque corrections derived fromtransducer and cabinet combined-volume changes must be included in thedT/dI_(a) sensitivity calculation (not shown).

Immediately following the pulse Sw_(1c) is closed to damp the rotorassembly 58 movement and return the rotor to its position at the startof the pulse. More rapid damping may be effected by using feedbackreturn through H_(1A) and Sw_(1d).

This measurement sequence of rotor assembly 58 angular positioning,pulsing, and damping is repeated each angular position .O slashed. stepat increasing levels of armature current I_(a) until sensitivitydecreases, for example 30 percent, to calibrate motor behavior at normaloperating levels, when approaching saturation, and when approachinglimits of usable rotation ±.O slashed.max. The rotational moment ofinertia of the rotor assembly 58 J is determined during apparatusfabrication and is used by the microcomputer 81 to calculate torque T ateach point in this process from angular acceleration T=J·d² .Oslashed./dt². These data, including the sensed maximum (saturation)rotational accelerations d² .O slashed./dt² and the armature currentsI_(a) required to produce them, are stored in memory of themicrocomputer 81 in an array H₂ (.O slashed.,I_(a)) organized by angularposition .O slashed. in steps of Δ.O slashed. as rows and armaturecurrent I_(a) as columns. H₂ (.O slashed.,I_(a)) is organized in muchthe same way as functions (H₂ -H₄) described hereinbelow. The torquesensitivity values entered in each cell in H₂ (.O slashed.,I_(a)) form a"map" of torque motor 23 torque-to-armature-current sensitivity dT/dIaat each point in its rotational position .O slashed. and armaturecurrent I_(a) range which account substantially completely for designstructure, variations in materials and manufacture, and aging of thefield magnets. This torque sensitivity map is normalized to produce atable of corrections V_(t1) =H₂ '(.O slashed.,V_(in)) to be added to theapplied audio signal V_(in) 82A so that the transfer function duringsound reproduction from the applied audio signal V_(in) 82A to torquemotor 23 torque T is linear.

Torque motor linearization--operation. In operation, Sw_(1d), SW_(2a)and SW_(2b) are open. At each sample period Δt the applied audio signalV_(in) 82 and rotor assembly 58 position .O slashed. from the encoder 24are read, stored normalization values looked up in H₂ '(.Oslashed.,V_(in)) in the linearization table, interpolation of storednormalization 5values performed if necessary, the applied audio signalV_(in) 82 modified by addition of correcting signals vtl derived from H₂'(.O slashed.,V_(in)) in microcomputer 81, converted back to analog formby the A/D-D/A converter 88, and fed to in e 80A.

This motor linearization technique may be applied to multipolecommutated motors to compensate motor magnetic circuit nonlinearities(pole geometry, ferromagnetic saturation) provided the commutation ispositionally consistent, as, for example when controlled by a digitalposition encoder. Δ.O slashed. increments in the H₂ (.O slashed.,I_(a))and H₂ '(.O slashed.,V_(in)) tables, in this case, would be very small.

Example pseudocode initial torque linearizing algorithms are: ##EQU2##

Movable vane damage protection--braking table creation. To produce adesired working bandwidth upper frequency of 80 Hz, motor maximum torqueT is high and the rotor assembly 58 structure is very light, with a lowrotational moment of inertia J. The potential for damage if movablevanes 71 and 72 collide with stationary vanes 62 and 63 is serious. Inconfigurations where the vanes are plane, e.g., FIGS. 2a and 3, the portangular width provides a safety margin for vane overtravel beyond ±.Oslashed.max, as vanes are not ordinarily driven beyond port boundaries.In the more efficient commercial configurations where vanes are likelyto have bent, twisted, or stepped configurations, and user- andcabinet-side ports overlap in the same angular sector about the chamberaxis 53, as described hereinbelow, e.g., FIG. 4, rotor assembly 58 andmovable vane 71 and 72 overtravel safety margins may vanish.

As part of the startup protocol a braking table is created which storesfor each rotor assembly 58 angular position .O slashed. the maximumvelocity which can be dissipated by the torque motor 23. Sw₀ and SW_(1d)are open, Sw_(1c) is closed, centering the rotor assembly 58. Thetorque-to-armature-current sensitivity data dT/dI_(a) in H₂ (.Oslashed.,I_(a)) derived as described hereinabove are processed using themaximum (motor saturation) accelerations recorded at each measuredrotational point .O slashed.. These are tabulated starting at the limitsof rotational position ±.O slashed.max and working toward the indexposition C. From these tabulated maximum accelerations the maximumangular velocity d.O slashed./dt at any rotational point .O slashed.which may be dissipated or decelerated to a stop at the limit of rotorassembly 58 travel ±.O slashed.max may be calculated by numericalintegration at each step Δ.O slashed. of increments Δd.Oslashed./dt=(2d² .O slashed./dt²· Δ.O slashed.)⁰.5 for each angularposition .O slashed. from ±.O slashed.max to C and stored as a column inthe braking table H₂ "(.O slashed.,1). The maximum braking signal V_(mb)on line 93 for the power amplifier 75 to produce this deceleration isalso calculated the power a gain A of the power amplifier 75, andentered as another column H₂ "(.O slashed.,2) in the braking table.

Movable vane damage protection--braking operation. At each sample periodΔt sensed rotor assembly 58 angular velocity d.O slashed./dt is comparedwith limit d.O slashed./dt values stored at H₂ "(.O slashed.,1) as afunction of .O slashed. in the braking tables by the microcomputer 81.If rotor assembly 58 angular velocity d.O slashed./dt is excessive, thatis, if it is above, for example, 85 percent of the maximum angularvelocity d.O slashed./dt for a given .O slashed. which may be dissipatedsafely before the limit of rotor assembly 58 travel ±.O slashed.max isreached, the applied audio signal V_(in) 82 is suppressed by openingSw₀, SW_(2d) is closed, and the appropriate decelerating signal V_(mb)read from table H₂ "(.O slashed.,2) by microcomputer 81 to provide thetorque T to brake or stop the rotor assembly 58 and movable vanes 71 and72 before contact with stationary vanes 62 and 63 can occur. The processis continued at each sample period until d.O slashed./dt is less thanthe H₂ "(.O slashed.,1) limit as f (.O slashed.), at which time Sw₀ isclosed and Sw_(2d) opened to resume normal operation.

Example pseudocode movable vane protection algorithms are ##EQU3##

Cabinet volume correction--initialization. In an economical domestic-usedesign, the volume displaced by the rotary acoustic radiator 22 is asignificant fraction of the volume of the supporting cabinet 26(typically 5-30%). At low frequencies air compliance within the cabinet26 provides the primary force resisting motor torque (inertial forcesdominate at high frequencies).

With changing angular position .O slashed. of rotor assembly 58,combined rotary acoustic radiator assembly 22 and cabinet 26 air volumecompliance changes. This nonlinear compliance is a source ofintermodulation and even-order harmonic distortion of sound output athigh levels; it is statically hyperbolic and dynamically adiabatic.Prudent use of acoustic damping materials within the cabinet 26 canrender the dynamic behavior substantially isothermal (hyperbolic). Thiscompliance must be defined for all positions .O slashed. of the rotorassembly 58.

As part of the startup protocol Swo is opened. Sw_(1d) is closed,providing fast position negative feedback centering the rotor assembly58. SW_(3a) is closed and the rotor assembly 58 is slewed through aseries of large angular deflections .O slashed. by voltages V₃ generatedat F₃ in microcomputer 81. Preferably these voltages are ramps of about0.1 sec duration terminating in plateaus rather than voltage steps inorder to limit percussive sound output during this part of the startupprotocol. Resulting angular positions .O slashed. are measured with theposition encoder 24. The feedback loop is permitted to settle (dV_(s)/dt and d² .O slashed./dt² less than some small constant)--V_(s) valueis not steady because the feedback loop will be compensatingintracabinet pressure changes due to leakage around the rotor assembly58 and the absolute value of armature current I_(a) and V_(s) will bechanging slowly.

The armature current I, required to hold these step positions isconverted to torque T using the torque-to-armature-current sensitivitiesstored at H₂ (.O slashed.,I_(a)) in the torque linearization table, andthence to intracabinet pressure P=T/(r★A_(v)). These data points,together with the known (from apparatus fabrication) rotary acousticradiator 22 displacement volume as a function of angular rotation .Oslashed. of the rotor assembly 58, dV=r★A_(v) ★d.O slashed., are used bythe microcomputer 81 to calculate the combined volume V of cabinet 26and the connected volume of rotary acoustic radiator 22 as a function ofrotor assembly 58 angular position .O slashed., fitting a curve of theform P₁ V₁ =P₂ V₂ ^(y) via least-squares or similar technique. Values atthe measured points are stored, along with the measured intracabinetpressure P in two columns of a PV table at H₃ (.O slashed.,1) and H₃ (.Oslashed.,2). P and V values as f(.O slashed.) not represented in thedata points are read from the fitted P and V curves and entered into theempty cells in H₃ (.O slashed.,1) and H₃ (.O slashed.,2). Finally, thedriving signal corrections V_(p) necessary to providepressure-compensating torque T are calculated for each .O slashed. andstored in the PV table at H₃ '(.O slashed.). Sw₀ is closed, Sw_(1c) andSw_(3a) are opened.

Cabinet volume correction--operation. During sound reproduction, Sw₀ isclosed, SW_(3b) is closed, and angular position data .O slashed. is usedas the argument to fetch driving signal V_(p) corrections V_(p) from H₃'(.O slashed.) to adjust motor torque T for the varying air pressure ofthe cabinet 26 and the connected volume of rotary acoustic radiator 22,producing a more linear response of the rotor assembly 58 to the appliedaudio signal V_(in) 82.

Example pseudocode cabinet volume correction algorithm is: ##EQU4##

A pressure-volume correction calculation similar to the above may beperformed if the cabinet is included during apparatus fabrication, butthe effective displaced intracabinet volume and damping characteristicsof acoustically absorptive materials are not usually well controlledduring manufacture and tend to change with time. The startup protocolcalculation described above will calculate and maintain accurate PVcorrections, even if not isothermal, over an extended range ofintracabinet volumes and apparatus life. It confers great latitude inapparatus placement, as in residential built-in and vehicle aftermarketinstallations, as the enclosure characteristics are determinedautomatically after installation.

Seal leakage correction--initialization. Reasonably effective seals areachieved by the very close approximation of surfaces about the marginsof rotor assembly 58 but air will leak. Leakage in the rotary acousticradiator assembly 22 varies as a function of internal geometry resultingfrom manufacturing tolerances, e.g., eccentricity of the axis of therotor assembly 58 within the enclosing cylinder sidewall 52 oreccentricity of the cylinder sidewall 52 itself, as might be expectedfrom an aging composite cylinder sidewall 52. This leakage is mostsignificant at low frequencies as the effective excursion of the rotorassembly 58 is reduced by the leakage (requiring that rotor assembly 58excursion be increased). Seal leakage is a source of distortion whichwould not be corrected by position- or velocity-derived negativefeedback, if used.

As part of the startup protocol Sw₀ is open, Sw_(1c) and Sw_(4a) areclosed, the rotor assembly 58 positioned at one limit of its rotation±.O slashed.max by a voltage V_(4a) from F_(4a) . SW_(1c) and Sw_(4a)are opened, Sw_(4b) is closed. The rotor assembly 58 is then rotatedslowly through its angular range, sweeping from one .O slashed.max limitto the other in such a way that the vane torque is in equilibrium withcabinet pressure as the rotor assembly 58 passes .O slashed.=0, by amicrocomputer-originated constant torque T, measured as a voltage dropV_(s) on sampling resistor 79, derived from a constant voltage v_(4b) atF_(4b) and adjusted by armature-current-to-torque corrections stored atH₂ '(.O slashed.,V_(in)) in the torque linearization table, whichconstant torque T corresponds in turn to a constant level ofintracabinet pressure P. Rotor assembly 58 angular velocity d.Oslashed./dt is converted to a leakage rate dV/dt = r · A_(v) · d.Oslashed./dt and recorded as a seal leakage table at H₄ (.O slashed.,T)in memory of microcomputer 81. This process may be repeated for severallevels of torque T, hence several levels of intracabinet pressure P, andstored in columns H₄ (.O slashed.,T) in the seal leakage table. Theseseal leakage rates are further converted into torque correction signalsV_(s1) as functions of .O slashed. and P and stored in columns at H₄'(.O slashed.,T) in the seal leakage table. Sw₀ is closed. Sw_(4b) isopened.

Seal leakage correction--operation. SW_(3b) and SW_(4c) are closed.Cabinet pressure P is estimated from rotor assembly 58 angular position.O slashed. as recorded in H₃ (.O slashed.,1), seal leakage torquecorrection signals V_(s1) are fetched from the seal leakage torque tablein microcomputer 81 memory at H₄ '(.O slashed.) and interpolated betweentable-stored H₄ '(.O slashed.) values if necessary. Motor torque T iscorrected to increase rotor assembly 58 angular velocity d.O slashed./dtto compensate for the estimated leakage rate by adding to the correctingsignal V_(s1) the applied audio signal V_(in) 82.

Example pseudocode seal leakage correction algorithms are: ##EQU5##

Bearing breakaway friction--initialization. Antifriction bearings havevery low static or breakaway (from stop) friction if operated with lightoil and without shields or seals. So operated they would be short-livedin the subject user environment, as user-environment air is dusty andpressure gradients can appear across the bearings. Grease lubricationand shields or seals are regarded as necessary, but they may increasebreakaway friction more than an order of magnitude. While bearings arestopped, this breakaway (static) friction is present and overall gainthrough the rotary acoustic transducer apparatus 21 is zero. Theconsequent dead zone or hysteresis is a significant source of distortionat low power (30-50 db below maximum) output. The phenomenon is mostserious when grease-lubricated ball bearings are stopped for more thanan instant, as the grease, pulled by capillary forces, tends to cuparound the balls. On first movement away from the stopped position thesecups must be penetrated by the balls, with resultant increased breakawaytorque needed to overcome the breakaway friction. Roller and needlebearings have similar properties.

As part of the startup protocol of this invention, these frictions aremeasured. Sw₀ is opened, Sw₈ is

closed, venting the cabinet so that cabinet pressure P does not apply atorque T to the vanes, and a programmed series of small signals isgenerated from a table of angular rotations (rows) and stop durations(columns) stored at F_(5a) in microcomputer 81. Typically the range ofrotations is from 0 to (360 degrees /(the number of balls in thebearings)), and the stop durations range from 0 to 50 milliseconds,typically in increments of 5 milliseconds. SW_(1d) is closed, invokingthe fast position negative feedback. Sw_(5a) is closed, accessing dataat F_(5a). A signal V_(5a) read from F_(5a) moves the rotor assembly 58and bearings a known angular distance .O slashed.. When they are broughtto rest, as determined by V_(s) =0 or armature current I_(a) =0 anddV_(s) /dt=0 or dI_(a) /dt=0, SW_(1d) is opened, interrupting the fastposition negative feedback. A stop-duration period is read from thetable in F_(5a) ; no signal is applied to the torque motor 23 duringthis period.

At the end of the stop-duration period, a breakaway-duration countert_(s) in microcomputer 81 operating at the sampling rate Δt is clearedand started, counting each sample period. SW_(5a) is opened and SW_(5b)is closed. A small ramp voltage-equivalent signal is generated in F_(5b)having a sign such that rotor assembly 58 will either reverse orcontinue its direction of rotation relative to that preceding the peakor plateau, which voltage-equivalent signal results in a smallincreasing torque T in the torque motor 23. When the bearing moves,indicated by incrementing of the position encoder 24, thebreakaway-duration counter is stopped and breakaway torque read as thearmature current I_(a) =V_(s) /R_(s) through sampling resistor 79 at themoment of breakaway and stored in memory of the microcomputer 81 in twobreakaway-friction tables at H₅ (.O slashed._(s),t_(s)), one each forbreakaway rotations which are continuations and those for reversals ofthe immediately prior rotation, each table organized by extent of priorangular rotation of the rotor assembly 58 prior to stop .O slashed.s andduration of stop t_(s). The values of .O slashed.s and t_(s) are roundedto integers to be used as subscripts and the torque stored in theappropriate H₅ (.O slashed._(s),t_(s)). This process is repeated untilthe desired range of prior rotation and stop duration has been tested.Because of the relatively coarse granularity of sampling period androtation measurement, tests at each point may be repeated and theresults combined statistically. During the breakaway measurement processthe system sampling rate may be increased to improve the resolution ofthe breakaway friction tables.

Data in H₅ (.O slashed._(s),t_(s)) are converted into voltage incrementsV_(bf) which will produce the desired breakaway torques and thesevoltage increments are stored in two tables H₅ '(.O slashed._(s),t_(s)),organized as to correspond with H₅ (.O slashed._(s),t_(s)). Sw₈, and Sw₀are closed, Sw_(1d), and Sw_(5b) are opened.

If prior velocity of rotation proves relevant for a given bearingstructure, seal structure, or lubricant, the friction-measuring processand the breakaway friction tables H₅ (.O slashed._(s),t_(s)) and H₅ '(.Oslashed._(s),t_(s)) may be extended to include velocity data as a thirddimension.

Typically during the mapping of breakaway friction the greater bandwidthof the fast position negative feedback function H_(1A) is used to reducerotary vane assembly 58 settling time. This closed-loop positioningsystem should be overdamped so that the bearings approach their stoppedpositions monotonically or the overtravel of bearing balls in the raceswill create channels in the lubricant beyond the stop position whichwould invalidate breakaway-friction measurements for plateaus in rotorassembly 58 motion.

Bearing breakaway friction--operation. Sw₀, Sw_(5c) are closed. When theapplied audio signal V_(in) 82 approaches a stop (indicated by small d.Oslashed./dt and opposing d² .O slashed./dt²) and rotation reversal orcontinuation at local waveform peaks or plateaus, the values of twoimmediate-history registers (three if velocity history is stored) inmicrocomputer 81 memory, retaining angular extent of rotation .Oslashed._(s) prior to stop, measured from the last stop, and theduration of stop t_(s) are updated. When the applied audio signal V_(in)82A requires resumption of rotor assembly 58 bearing motion as indicatedby dVs/dt≠0 the previously recorded immediate-history registers are readto provide the arguments .O slashed._(s) and t_(s) as subscripts toaccess the breakaway-friction tables which are read from H₅ '(.Oslashed._(s),t_(s)) in microcomputer 81 memory and V_(bf) added to theapplied audio signal V_(in) 82D to produce a momentary (one sampleperiod duration) armature current pulse Ia which substantially reducesor overcomes bearing breakaway friction, minimizes hysteresis and deadzone, and reduces output distortion.

Breakaway torques are reproducible, influenced by temperature as well asby extent (and sometimes velocity) of rotation immediately preceding astop, and by duration of the stop. After the startup protocol, breakawaytorques are recorded selectively during sound reproduction at the momentof breakaway and recorded in the breakaway friction table H₅ (.Oslashed._(s),t_(s)) of the microcomputer 81 as function of the priorextent of rotation .O slashed._(s) and duration of stop t_(s), and alsoheld in the immediate-history registers described hereinabove. Asdescribed hereinabove these parameters, suitably rounded to serve assubscripts, are used to store data in H₅ (.O slashed._(s),t_(s)) as wellas fetch it from H₅ '(.O slashed._(s),t_(s)). In operation twoadditional immediate-history registers store armature current I_(a) androtor assembly 58 angular position .O slashed. at the moment of rotorassembly 58 stop. New data are updated in H₅ (.O slashed._(s),t_(s))only if armature current I_(a) and rotor assembly 58 angular position .Oslashed. are small, typically 5 percent of I_(a) maximum and .O slashed.maximum, respectively, implying small confounding drive and intracabinetpressure torques which must nevertheless be subtracted respectively frombreakaway-friction torque values to be entered in H₅ (.Oslashed._(s),t_(s)). Breakaway-friction table values in H₅ '(.Oslashed._(s),t_(s)) are updated from data in H₅ (.O slashed._(s),t_(s))as simple moving averages calculated over (say) 50 events having thesame immediate-history register prior rotation .O slashed.₅ and stopduration t_(s) values, thus reflecting bearing breakaway frictionchanges. The event count must be recorded as additional data for H₅ (.Oslashed._(s),t_(s)) cells, most conveniently in a similar array, toassure that the accumulated data are statistically valid prior toupdating working H₅ '(.O slashed._(s),t_(s)) table data. Updatedbreakaway friction tables are used as described above.

As an alternative or adjunct to the breakaway friction managementdescribed above, to limit breakaway friction buildup during rotorassembly 58 bearing stops described above, the normal stop-pause-startrotor assembly 58 sequence at waveform peaks and plateaus of the appliedaudio signal V_(in) 82A is forestalled by not permitting the bearingrotation to pause. When the applied audio signal V_(in) 82A would causethe bearings to pause, detected by microcomputer 81 as a low angularvelocity d.O slashed./dt and opposing d² .O slashed./dt², themicrocomputer 81 closes Sw_(5c) and generates a infrasonic voltage inputV_(5c) at F_(5c) sufficient to cause the rotor assembly 58 and thebearings to continue to move slowly, limiting grease-cup formation inthe bearing races. This infrasonic voltage from F_(5c) is reversed fromtime to time to limit accumulation of rotor assembly 58 angulardisplacement .O slashed. from this source to a few degrees. Abreakaway-reducing torque derived from H₅ '(.O slashed._(s),t_(s)) maybe used to assist this rotation reversal. When applied audiosignal-generated movement is resumed, indicated as a d.O slashed./dtother than that produced by F_(5c), Sw_(5c) is opened and the infrasonicinput at F_(5c) suppressed. Thus the bearings either accelerate from analready-moving state or reverse instantaneously. In the first casestatic or breakaway friction is nearly eliminated. In the second case itis stabilized and minimized. The slow movement decreases availableapparatus volumetric efficiency, but the decrease may be substantiallyrecovered as described hereinbelow.

Example pseudocode breakaway friction algorithms are: ##EQU6##

To facilitate breakaway-friction management a delay interval of a fewmilliseconds from the delay buffer 89, which may be a circular buffer inmicrocomputer 81 memory as in FIG. 2a or implemented separately insupporting hardware as in FIG. 2b, may be applied to the applied audiosignal V_(in) 82D before input via line 91 to the functions inmicrocomputer 81 and subsequent transmission to the power amplifier 75and torque motor 23. Such a delay would-compromise operation of amiddle- or high-frequency range loudspeaker with other loudspeakers inthe same system, as a delay of each millisecond is a phase shift ofabout 100 degrees at 300 Hz and increases with frequency. But sources oflow-frequency sounds difficult for a user to locate and wide latitude isavailable in subwoofer placement within a typical residential room orvehicle (below 100 Hz. 2 milliseconds is less than 30 degrees). A delayof 2 milliseconds, the nominal value in this invention, is roughlyequivalent to moving the subwoofer 2 feet away from the user, anegligible amount. The "preview" afforded by microcomputer 81 samplingof the applied audio signal V_(in) 82 before passage through the delaybuffer 89 permits antifriction processing to be more easily managed,e.g., if vane assembly 58 rotation, despite low d.O slashed./dt andopposing d² .O slashed./dt², is not in fact going to stop because theacoustic waveform is not approaching a local peak or plateau, none ofthe antifriction techniques above need be applied, and computerprocessing cycles may be diverted to other functions such as updating orsmoothing table data. If indeed rotation is approaching a local peak orplateau and will stop, the antifriction processes may be invoked. Ifappropriate, the delay permits computer data sampling period Δt to bedecreased near peaks and plateaus to improve antifriction dataresolution.

When the preview is used, the reproduced sound is taken from the delayedsignal on line 91, and the antifriction decision-making data are fromthe undelayed signal on line 82D. In order for the antifriction sensingto be properly phased with rotation of the rotor assembly 58, line 82Dmust feed a digital filter within microcomputer 81 which has, to areasonable approximation, the same gain and phase characteristics as theopen-loop control system including microcomputer 81, amplifier 75, motor23, and position encoder 24. This should include at least an integratormiming the motor 23 and rotary acoustic radiator 22 and additional polesmatching the motor and armature poles if they lie in the workingfrequency band of the apparatus.

Torque motor cooling--initialization. A rotary acoustic radiator 22formed of metal is a potentially efficient heat sink for the torquemotor 23. Torque motor-heated air is cyclically swept into the cylinder51 from a cabinet 26 with each cycle of the rotor assembly 58. Thisairflow is turbulent, hence effective in transferring torque motor 23heat to the cylinder sidewalls 52. Though heat transfer through themovable vanes 71 and 72 to the outside environment is limited by theirlightweight internal structure, the stationary vanes 62 and 63 may havehigh conductivity to conduct heat through themselves and to conduct heatto the cylinder sidewalls 52 and end walls 54 and 56, which cylinderwalls 52 and end walls 54 and 56 are being swept alternately by air fromthe cabinet 26 and outside air. When cylinder 51 is provided withcylinder sidewall 52 user-side ports and cabinet-side end wall ports ashereinafter described, the cylinder 51 outside walls may be swept bypumped air suitably deflected from the user-side ports. The stationaryvanes 62 and 63 and cylinder 51 outside walls may be provided with fins(not shown) to provide increased surface for heat transfer anddissipation. In a large unit (750 cu. in. displacement/stroke), torquemotor heat of 300 watts at full power output may be transferred anddissipated with an air temperature rise of less than 5 degrees C.Smaller units are more easily cooled because of their highersurface-to-volume ratio.

The nominal ambient temperature torque motor armature winding resistanceis determined during the startup protocol as follows. Sw₆ is closed. Themicrocomputer 81 generates a test voltage which is applied to thearmature to measure R_(a) =(V_(d) -V_(s))·R_(s) / V_(s). This voltagemust be small enough that significant motor 23 counter emf is notgenerated, and if alternating, of angular frequency well below R_(a)/L_(a), the armature pole.

Torque motor cooling--operation. In normal operation, for a givenacoustic output, air velocity into cabinet 26, particularly if directedthrough end ports as in FIG. 7 hereinbelow, is adequate to cool thetorque motor 23, as air velocity is constant independent of frequency.If deadspace between rotary acoustic radiator cabinet-side ports, as forexample 123-and 124, and the armature of the torque motor 23 is keptsmall, cooling of the torque motor 23 in the upper end of its frequencyrange is also adequate. At lower frequencies deadspace is lessimportant, as displaced air volume per cycle is larger. However, in atransition from extended loud to soft sound passages, stored armatureheat must be dissipated or distributed into the torque motor 23 mass.When the microcomputer 81 detects such an average audio signal leveltransition as a decrease of average absolute value of rotor assembly 58displacement .O slashed. below a defined threshold such as 10 percent of.O slashed.max, and the armature temperature is high, e.g., more than 45degrees C. above ambient, as indicated by a rise in armature resistanceR_(s) of about 15 percent above nominal, the microcomputer 81 closes Sw₆and produces an infrasonic signal at F₆ (typically 3 Hz) which producesin turn a vane movement of 10 to 15 percent of maximum angulardeflection ±.O slashed.max sufficient to dissipate accumulated armatureheat in the absence of large output. The infrasonic signal at F₆ iscontinued until the armature temperature is reduced to a specifiedtemperature above ambient, typically 20 degrees C., as determined byperiodic measurement of armature resistance R_(a) =(V_(d) -V_(s))·R_(s)/V_(s) by the microcomputer 81. The time required for temperaturereduction is typically several minutes. This cooling techniquepotentially reduces maximum available volumetric efficiency somewhat butoffers substantially silent dissipation of accumulated torque motor 23heat without cost or noise of additional equipment. This reduction canbe substantially obviated as described hereinbelow.

Example pseudocode cooling algorithms are: ##EQU7##

Bearing wear distribution--operation. Bearings used for oscillatingloads tend to localize wear, as the load-bearing operating points ofbearing surfaces are most often near the index position C. Antifrictionbearings may concentrate wear unevenly over the surface of their racesand rolling elements (balls, needles, rollers). In the presentinvention, which typically uses ball bearings, the microcomputer 81closes Sw₇ and generates at F₇ a low amplitude random infrasonic voltageV₇. This infrasonic signal V₇ continuously shifts the rotationaloperating point of the rotor assembly 58 about the index position C.

If bearing rolling elements have small diameters this movement issufficient to assure that said rolling elements roll over in their racesevery few minutes, improving wear distribution. This slow wanderingreduces maximum available volumetric efficiency. This reduction can besubstantially obviated as described hereinbelow. An example pseudocodebearing wear distribution algorithm is: ##EQU8##

Fast Centering--operation. Reductions of maximum volumetric efficiencydue to functions which manage bearing breakaway friction, torque motorcooling, and bearing wear may be consolidated. Together they reducemomentary maximum acoustic output no more than 1.5 db, not normallydetectable by the user. In the presence of high level applied audiosignal V_(in) on line 82A producing angular rotations .O slashed. ofmore than 50 percent of .O slashed.max, the duration of this smallmaximum output reduction is limited to a fraction of a second as themicrocomputer 81 centers the rotor assembly 58 operating point on theindex operating point C for the duration of the loud passage and for ashort time, typically 30 seconds, thereafter, by suppressing the bearingbreakaway friction, torque motor cooling, and bearing wear managementsignals and closing the fast centering feedback loop. This is done byopening Sw_(5c), Sw₆, and SW₇, and momentarily (typically 0.1 second)increasing the bandwidth of the centering position feedback system byopening Sw_(1c) and closing SW_(1d). The system is returned to normal,with Sw_(5c), Sw₆, and Sw₇ closed when .O slashed. has remained below 30percent of .O slashed.max for 30 seconds.

An example pseudocode fast centering algorithm is: ##EQU9##

From the foregoing it can be seen that the rotary acoustic transducerapparatus 21 may include microcomputer control of movable vanecentering, movable vane rotation limiting, motor cooling, and bearingwear. Microcomputer 81 functions also may include calibrating andcorrection of torque motor nonlinearity, air compliance nonlinearity,vane edge leakage, and bearing breakaway friction. With the exception ofthe centering system, these functions should all be distinguished fromnegative feedback control, commonly defined as "control by sensing ofthe controlled variable" and usually implying concurrent sensing andcontrolling in the same frequency band as the useful bandwidth of thedevice, in this case from about 10 to 160 Hz. The useful frequencyresponse capability below and above the 20 to 80 Hz nominal workingbandwidth contributes to well-controlled user-perceived frequencyresponse of the apparatus. Negative feedback is commonly used to reducedistortion of audio devices and commonly associated with audibleresponses to input transients which include transient frequencies notfound in the input source signal. In this apparatus centering systemnegative feedback loop frequency upper limits are far belowuser-perceivable frequencies, usually below 1 Hz.

The control approach in this invention has two broad parts: Apparatusmanagement, and calibration, rotation limitation, and forward errorcorrection. Apparatus management functions comprised of centering, motorcooling, and bearing wear are implemented at infrasonic (i.e., below 20Hz) frequencies well below audible frequencies and therefore notperceivable to the user. Calibration, rotation limitation, and forwardcorrection functions, which are implemented in and above the usefulbandwidth of the apparatus, use corrections derived from historical dataaccumulated and stored in tables in microcomputer 81 memory added to theapplied audio signal V_(in) 82D in such a fashion that the sound outputof the apparatus is corrected for major sources of nonlinearity,resulting in high level of output linearity, i.e., low distortion.

The use of negative feedback is not forestalled by forward errorcorrection. Rather the effect of the extensive forward correction is toreduce considerably, perhaps 10 to 20 db, the amount of negativefeedback needed, if indeed it is used, to achieve a given level ofoutput distortion.

The rotary acoustic radiator assembly 22 of the present invention issuited for reproducing sounds in the nominal working bandwidth of 20 to80 Hz, acting much like a point source, as the dimensions of a largemodel of a rotary acoustic radiator, in particular the distance betweencenters of ports 64 and 66 in the first or top end wall 54 (e.g., about9 inches), are small relative to the wavelength (approximately 14 feet)of the highest frequency to be reproduced.

With the rotary acoustic radiator assembly 22 mounted in a cabinet 26 asshown in FIG. 1 with the ports 31 and 32 in the cabinet 26 inregistration with the ports 64 and 66 in the first end wall 54, airmovements in cabinet ports 31 and 32 at low amplitudes fuse into asingle acoustic pressure wave within a relatively short distance fromthe apparatus 21 even if a diffuser-attenuator is not used. At highamplitudes port exhaust plume air velocities for the ports 31 and 32 mayapproach 100 miles per hour in a rotary acoustic radiator assembly 22displacing less than 750 cubic inches of air per stroke. This exhaustplume, which unmodified would create a distributed sound source, isslowed in a diffuser-attenuator as shown in FIG. 1 to transform plumekinetic energy into potential energy to thereby cause fusion of the portexhaust plume energy into a single acoustic wave within a relativelyshort distance from the apparatus 21.

Typically there are no significant compliant members linking thestationary and moving parts of the rotary acoustic transducer apparatus.The rotor assembly 58, the torque motor 23, and the position sensingmeans 24 are rotatable components which are balanced about thelongitudinal axis 53 so that there are no net positioning forces to movethem from any rotational position. If mounted in a literally "infinitebaffle," the rotatable components are without a low-frequency resonanceand may be easily driven to frequencies below 1 Hz. In practice the sizeof a cabinet 26 and the compliance of the air within a cabinet 26,together with the movable vane area and the moment of inertia of allrotating components combined, set the low-frequency resonance of theapparatus 21. As noted hereinabove, typically this resonance is at 3 to6 Hz, well below the audible frequency spectrum.

Volumetric efficiency, which is the percentage of the rotary acousticradiator assembly 22 internal volume which may be swept by the movablevanes 71 and 72 in a single stroke, is limited as shown in FIGS. 2a, 2band 3 by the thickness of movable vanes 71 and 72, stationary vanes 62and 63, and the areas of ports 64 and 66 and 67 and 68.

For the apparatus 22 the volumetric efficiency is about 60 percent,which compares favorably with less than 15 percent for a conventionallow-frequency loudspeaker cone assembly. Nevertheless the largestpossible volumetric efficiency is desirable, both to increase acousticoutput and for certain other apparatus management and distortionreduction purposes described hereinbefore. Vane thicknesses are smalland difficult to reduce. Reduction in port areas increases volumetricefficiency at the expense of increased port air velocities.

FIG. 4 shows an alternative rotary acoustic radiator assembly 101similar to that of FIG. 3 except that the movable vanes 102 and 103attached longitudinally at the hub 59 are formed--twisted--so that theouter margins are offset or tilted clockwise in an upward directionrelative to the longitudinal axis 53 of the rotary acoustic radiator101. The vane deflection at the cylinder sidewall 52 is roughly equal tothe port width at the sidewall 52, causing the movable vanes 102 and103, when rotated to their extreme counterclockwise positions, tosubtend the same angular space about the longitudinal axis 53 as theports 67 and 68. For example, if rotor assembly 58 is rotatedcounterclockwise to its extreme position, the upper margin of movablevane 102 will approach the upper margin of stationary vane 62 while thelower margin of movable vane 102 approaches the left margin of port 67as viewed in FIG. 4. Thus the movable vanes 102 and 103 travel throughsubstantially 180 degrees, less the total of the angles subtended by theport width, the thickness of the movable vanes 102 and 103, and thestationary vanes 62 and 63. Assuming that the port width is 30 degreesand the vanes in total subtend 15 degrees, this has the same effect onvolumetric efficiency as removing one port set, and provides a gain involumetric efficiency to about 75 percent without changing the port airvelocity from that of the embodiment shown in FIGS. 2 and 3.

In FIG. 5 there is shown another embodiment of the rotary acousticradiator assembly of the present invention. The radiator 107 achievesapproximately the same volumetric efficiency gain as the radiator 101 inFIG. 4 by forming--twisting--the stationary vanes 108 and 109 ratherthan the movable vanes 71 and 72 so that their inner margins areparallel to the longitudinal axis 53 and their outer margins are offsetor tilted clockwise in an upward direction along the cylinder sidewall52. This embodiment brings the ports substantially into longitudinal orvertical alignment in the same angular sectors about the axis 53, thatis, ports 64 and 67 are longitudinally or vertically aligned and 66 and68 are as well. The same port air velocities and volumetric efficiencyare achieved as in FIG. 4.

The air velocities through the relatively small radial ports shown inthe embodiments in FIGS. 2 through 5 associated with high volumetricefficiency can be high, increasing requirements that thediffuser-attenuator, such as the diffuser-attenuator 36 shown in FIG. 1,slow the airflow presented to the user environment. Increased airvelocity on the cabinet side of the apparatus due to reducedcabinet-side port area may be advantageous in affecting heat transferfrom the driving torque motor 23.

Increased cabinet-side port air-turbulence noise is not a significantproblem because such noise can be absorbed by acoustic material (notshown) provided within a cabinet on the walls of said cabinet.Decreasing user-side port air velocity by increasing user-side port areareduces port turbulence noise.

An embodiment of the rotary acoustic radiator assembly utilizing theseprinciples is shown in FIG. 6. The rotary acoustic radiator assembly 111has stationary vanes 114 and 115 formed much like those shown in FIG. 5,but the overall deflection along the cylinder sidewall 52 isaccomplished by a localized bend in stationary vanes 114 and 115.User-side ports 112 and 113 are provided in the cylinder sidewall 52 toincrease total user-side port area and to reduce user-side port airvelocity and noise. The user-side ports 112 and 113 are disposed nearthe upper extremity of the side wall 52 adjacent the stationary vanes108 and 109 and overlie the ports 67 and 68.

In FIG. 7, in another embodiment of a rotary acoustic radiator assembly116, stationary vanes 117 and 118 are formed in a Z-shape in crosssection normal to the longitudinal axis 53 which permits full lengthuser-side ports 121 and 122 to be provided in the cylinder sidewall 52,providing the lowest user-side port air velocity for a given anglesubtended by ports and vanes. The cabinet-side ports 123 and 124 arereduced in area, but are well positioned to deliver cooling air to thetorque motor 23. Ports 126 and 127 in the top wall 54 are reduced inarea as shown to accommodate the Z-shaped stationary vanes. However, itshould be appreciated that radial-sector-shaped full area ports, asshown in FIGS. 3 through 6, may be provided in the top wall 54 bysomewhat more complex formation of the stationary vanes (not shown). Thecombined end and side porting on the user side and end porting on thecabinet side, as shown in this FIG. 7, produces minimum user-sideairflow velocity for a given displacement and volumetric efficiency andhence the quietest user-perceived airflow.

In the embodiment of the rotary acoustic radiator 131 shown in FIG. 8,the stationary vanes 132 and 133 have a different form, user-side andcabinet end wall ports have been eliminated and user-side rectangularports 134 and 136 and cabinet-side rectangular ports 137 and 138 areprovided in the cylindrical sidewall 52. The stationary vanes 132 and133 have upper offset portions 132a and 133a and lower offset portions132b and 133b and intermediate adjoining portions 132c and 133cextending at right angles thereto. The ports 134 and 137 are in verticalalignment as are the ports 136 and 138. The user- and cabinet-side portsthus subtend the same angular sector about the longitudinal axis 53 ofthe radiator 131. The user-side ports 134 and 136 may have a verticaldimension along the axis 53 which is greater than that of thecabinet-side ports 137 and 138. Like the embodiments in FIGS. 4, 5, 6and 7, this is a high volumetric efficiency embodiment.

The embodiments of FIGS. 4, 5, 6, 7 and 8 have in common vanes which maybe simply described as "bent" in one or more planes or twisted. Allvanes have two faces exposed to the air flow with dimensions largerelative to their thickness. Though, as shown, movable vanes taper inthickness linearly from root to tip and stationary vanes do not, avariety of other thickness profiles is plausible for either movable orstationary vanes, e.g., exponential. The bent or twisted vanes have incommon the geometric attribute that if a large number of lines areprojected from points distributed uniformly over one face of a bent ortwisted vane through the interior of the vane to the nearest point onthe second face of the vane, and if the midpoints of all such lines aremarked, said midpoints will not lie in a single plane, that is, thesurface which is the locus of said midpoints is non-planar.

The edges of the ports in the embodiments hereinbefore described may beprovided with smooth aerodynamic surfaces to promote laminar flowthrough the ports and minimize port flow resistance and generation ofair turbulence noise which has predominantly high frequency components.

The cabinets for the embodiments of the radiators shown in FIGS. 2through 5 may be of the same type as cabinet 26 shown in FIG. 1. Thediffuser-attenuators also may be of the same type as diffuser-attenuator36 shown in FIG. 1. The embodiment of the radiator 111 shown in FIG. 6requires that it be raised through the cabinet top wall 28 (see FIG. 1)sufficiently to expose the sidewall ports 112 and 113. Thediffuser-attenuator 36 also would be raised by a similar distance.

The rotary acoustic radiator 116 in FIG. 7 must be outside the cabinet,as for example on top of the cabinet top wall 28 of the cabinet 26 inFIG. 1, with cabinet-side ports 123 and 124 in registration with cabinetports 31 and 32.

The rotary acoustic radiator 131 of FIG. 8 is supported in the cabinettop wall 28 so that the user ports 134 and 136 are above the cabinet topwall 28 and the cabinet side ports 137 and 138 are below the cabinet topwall 28.

All of the rotary acoustic radiator assemblies hereinbefore describedhave a construction which makes it easy to mount them in or on cabinets.Also, the cabinets can be of various shapes and sizes. For example,instead of a rectangular cross section, cabinets can have a circular orelliptical cross section, or they may be part of vehicular coachwork.The rotary acoustic radiator assemblies may also be operated withoutcabinets, as when mounted in the ceiling of a room, or in the coachworkof a vehicle so the cabinet- or back-side ports vent outside thevehicle.

A more detailed embodiment of a sound reproducing apparatus or rotarytransducer apparatus incorporating the present invention is shown inFIGS. 9 and 10. The apparatus is comprised of a rotary acoustic radiator201 comprising a cylindrical housing 202. The housing 202 as shown hasbeen machined from an aluminum casting to provide a radially ribbedfirst end wall or top wall 203 of 0.5 inch nominal thickness which isintegral with a nominal 0.5 inch cylindrical side wall 204. A radiallyribbed bottom or second end wall 206 of nominal 0.5 inch thickness issecured to the side wall 204 by suitable means such as cap screws (notshown) extending through holes 207. By way of example, the enclosed orcylindrical volume 209 within the housing 202 can have a suitable size,for example a 14 inch diameter with a height of 8 inches. The top wall203 is provided with two radially extending sector or pie-shapeduser-side ports 211 and 212 whereas the bottom wall 206 has similarcabinet-side ports 213 and 214, each of the ports subtending a suitableangle such as 30 degrees. Although the housing 202 can be mounted in anydesired orientation, it is generally preferable to have the housingoriented vertically in which the first or top wall 203 overlies thesecond or bottom wall 206.

Radially extending stationary diametrically opposed vanes 216 and 217are positioned within the cylindrical volume 209. These vanes 216 and217 can be formed of 0.375 inch aluminum plate, and extend from the topwall 203 to the bottom wall 206.

A rotor assembly 218 is provided within the housing 202 and has acentral axially extending shaft 219 formed of a suitable material suchas aluminum. The upper end of the shaft 219 is rotatably mounted in asuitable bearing such as a sealed ABEC class 7 (very low noise) ballbearing assembly 221 mounted in the top wall 203. The rotor assembly 218is constructed of materials so as to be relatively light in weight andso as to provide a low inertial mass. A hub 222 is mounted on the shaft219 and has diametrically opposed radially extending vanes 223 and 224which are secured to the hub 222. The vanes 223 and 224 are tapered incross section in a radial direction to provide greater strength at theinner margins or roots 223a and 224a of the vanes 223 and 224. By way ofexample, the vanes 223 and 224 can have a thickness of 3/4 inch at theroots 223a and 224a and approximately 3/8 inch at the distal margins ortips 223b and 224b. The hub 222 (e.g., 4 inches in diameter) can beformed of annealed Kevlar (trademark) foam. The movable vanes 223 and224 can be formed of 4.5 lb. per cubic foot aluminum honeycomb coreepoxy bonded to a very thin (0.003 inch) aluminum skin which forms thevane faces. Bonded skin doublers reinforce the roots 223a and 224a. Inthis way it is possible to distribute the stresses outwardly from thehub 222 towards the distal margins of the movable vanes 223 and 224through the movable vane facing material.

Thus, it can be seen that the present invention in FIG. 9 utilizes twosets of at least two vanes each, one set of which is stationary, i.e.,the vanes 216 and 217, and one set of vanes 223 and 224 each of which ismovable. The smallest desirable number of vanes in each set is twobecause when they are diametrically aligned this number balances thehigh centrifugal forces generated in each of the vanes, which canceleach other in the two vanes. The two sets of vanes have approximatelythe same area.

An electric drive or torque motor 226 which is much like a larged'Arsonval galvanometer is provided for driving the rotor assembly 218in a rotary reciprocating motion through a 110 degree maximum arc. Thetorque motor 226 consists of an outer housing 227 formed of a suitablematerial such as low carbon steel. The housing 227 is in the form of acylinder or ring which has an outer diameter which corresponds to theouter diameter of the housing 202. The outer housing 227 serves as themain support for the other motor components and as a flux return path.

A plurality of five serially connected surfaces 231 are machined intoopposite sides of the interior of the housing 227 and are separated by asuitable distance, as for example approximately 3 inches at theextremities of the same. The two series of five surfaces 231 serve toform a decagon which can be considered to be cut in half and separatedby the 3 inches hereinbefore described. Mating flux concentrating polepieces 236 and 237 formed of a similar material such as low carbon steelare disposed within the housing 227 and are provided with seriallyconnected planar surfaces 238 which face the corresponding seriallyconnected planar surfaces 231. Magnets 239 of a suitable material suchas ferrite having trapezoidal upper faces are mounted between the polepieces 236 and 237 and the surfaces 238 thereof, and the surfaces 231 ofthe housing 227. Thus, five magnets 239 are provided for each half ofthe decagon to provide a dipole field. The magnets can be of a suitablesize, as for example 4 inches wide by 6 inches high by 1 inch inthickness, and have their sides beveled as shown in FIG. 9 so they fitclosely to each other between the surfaces 231 and 238. The magnets 239can be held in place in a suitable manner such as by bonding the same tothe surfaces 231 and 238 by a suitable adhesive. In addition, safetypins (not shown) formed of a suitable material such as 7075 aluminum canextend diametrically from the flux concentrators 236 and 237 through themagnets 239 and be secured to the housing 227 to further ensure that themagnets are retained in their proper locations. By way of example, fourof such pins can be provided which are spaced 90 degrees apart. The polepieces 243 and 244 and flux concentrators 236 and 237 and 241 and 242concentrate the magnet flux by approximately 2 1/2 times to provide atotal flux in gaps 251 and 252 of approximately 6 kilogauss. Pole pieces243 and 244 are separated to reduce flux leakage and to ensure the fluxpasses through the appropriate portions of the drive motor 226.

It should be appreciated that it is possible to utilize moresophisticated magnetic materials, for example neodymium-based ferritemagnets, which may make it possible to eliminate the use of the fluxconcentrators 236 and 237 and thereby substantially reduce the size ofthe motor. However, because of the lower cost of the conventionalferrite material, ferrite material has been utilized in the torque motor226 shown in FIG. 9.

Additional semi-circular flux concentrators 241 and 242 are providedwhich lie adjacent the flux concentrators 236 and 237. The semi-circularpole pieces 243 and 244 of soft iron are held in spaced apart positionsby T-shaped support plates 246 and 247 formed of a suitable materialsuch as aluminum and engaging opposite extremities of the pole pieces243 and 244. The T-shaped support plates 246 and 247 also support acylindrical core 249 of very pure iron of a suitable size, as forexample 3 1/2 inches in diameter and 4 inches in length. Thus, there areprovided a pair of semi-circular spaces 251 and 252 between the polepieces 243 and 244 and the central core 249.

A rotatable armature rotor 256 (see FIGS. 9 and 10) is disposed in thespaces 251 and 252 and is mounted upon the shaft 219 which extendsthrough a hole (not shown) in the central core 249. The armature iscomprised of spaced apart parallel legs or saddles 257 (see FIG. 10)that are U-shaped in cross section and which have "spiders" 258 and 259disposed on opposite ends thereof but which are spaced therefrom by gaps261 so that the spiders 258 and 259 are insulated from the saddles 257.

The top spider 258 is provided with a diamond-shaped central hub 262which is adapted to be mounted on the shaft 219. The hub 262 is disposedbetween upstanding sidewalls 263 formed integral with a plate (notshown) that carries the hub 262. The sidewalls 263 bulge outwardly in abroad "V". The lower spider 259 is also provided with a hub 266 and ismounted on and is supported by four radially extending spring spokes 267adjoining arcuate crosspieces 268 at their distal extremities. Thecrosspieces 268 have mounted thereon depending V-shaped structures (notshown) which abut the hub 266 to provide a diamond-shaped structuresimilar to the diamond-shaped hub 262. The spider 259 is also providedwith upstanding spaced apart sidewalls 271 in the same manner assidewalls 263 which also bulge slightly outwardly in a broad "V".

The saddles 257 and the spiders 258 and 259 are supported in a jig (notshown) to provide appropriate spacing, and then have wound thereon aninsulated conductor to provide a winding 272. The jig is then removed.The armature rotor 256 preferably has a length which is greater than itswidth, as for example it can have a length of 6 inches and a width of 41/2 inches, and is wound with a suitable conducting wire such as16-gauge insulated aluminum wire. The wire is wound so that it extendsover the spiders 258 and 259 on opposite sides of the hubs 262 and 266between the spaced apart V-shaped sidewalls 263 and 271 and into thesaddles 257. By way of example, in one embodiment of the inventionapproximately 390 turns were utilized in the armature rotor winding 272.The conducting wires are then held in place by a fiberglass-filledepoxy. The sidewalls 263 and 271 serve to prevent the winding 272 fromspreading apart, whereas the diamond-shaped structures associated withthe hubs 262 and 266 serve to prevent the windings from collapsinginwardly.

The spring spokes 267 of the spider 259 are approximately 1000 timesmore compliant axially than they are tangentially. They serve totransfer torque from the winding 272 to the hub 266 while being able toflex longitudinally of the torque motor rotational axis as the winding272 expands and contracts with changing power input without having anytendency for the armature legs 267 to bow outwardly and touch the polepieces 243 and 244. The thin (0.014 inch) longitudinal aluminum saddles257 are bonded to the inner and side surfaces of the legs 267 of thearmature winding 272 to stiffen the armature winding 272 against lateraldeflection during acceleration.

The torque motor housing 227 is secured to the bottom plate 206 bysuitable means such as four through bolts (not shown) spaced 90 degreesapart and extending through holes 276 in the housing 227. The shaft 219extends beyond the armature rotor 256 and is rotatably mounted in alower bearing assembly 277 and of the same type as bearing assembly 221.The bearing assembly 277 is mounted in a carrier 278 which is secured tothe end wall of a bearing support housing 279 secured to the lower facesof the pole pieces 243 and 244 by bolts (not shown).

A position encoder 286 is mounted on the shaft 219 and is secured to thelower bearing housing 279. The position encoder 286 senses rotation ofthe armature 256 which can travel through a suitable angle, as forexample 110 degrees. It has a resolution of about 125 bits per 10 degreeof rotation. The information from the position encoder 286 can beutilized for providing velocity information or acceleration informationfor controlling the torque motor 226.

Power is supplied to the armature 256 through a capstan 291 mounted onthe shaft 219 above the bearing 277. The capstan 291 is formed of asuitable insulating material such as Delrin. Flexible conducting foilstrips 292 and 293 formed of a suitable spring-like, fatigue-resistantconducting material such as a 0.004 inch thick beryllium copper eachhave one of their ends mounted in spaced apart separate slots in thecapstan 291. The strips 292 and 293 are connected by leads 294 and 295which extend from the capstan 291 and are connected to opposite ends ofthe winding 272 of the armature 256. At rest, the strips 292 and 293each subtend approximately 90 degrees on the capstan 291 and areconnected to conductive fishing pole-like tensioning leaf springs 296and 297 about 1.5 inches long, formed of thicker (0.018 inch) berylliumcopper. The leaf springs 296 and 297 are mounted in insulating supportblocks 298 mounted in the lower bearing housing 279. Leads 299 connectedto the leaf springs 296 and 297 extend from the support blocks 298 andare connected to the microcomputer-adjusted applied audio signal fromthe power amplifier. The leaf springs 296 and 297 have sufficient lengthso that they can accommodate somewhat more than the 110 degreesreciprocating rotational movement of the armature 256 as it travelsthrough its maximum excursions.

The operation of the embodiment of invention shown in FIGS. 9 and 10 isvery similar to that hereinbefore described in conjunction with theembodiment shown in FIGS. 1 and 2. The movable vanes 223 and 224 whichare fastened to the shaft 219 and hub 222 are driven in rotaryreciprocation that follows the microcomputer-adjusted audio signalapplied to the torque motor 226. The distal margins 223b and 224b of themovable vanes 223 and 224 are very close to the internal wall of thehousing 202 so that as they move they sweep substantially all of theradial projection of the housing 202 excepting the area of the shaft 219and hub 222 at any point in their rotation. The inner margins of thestationary vanes 216 and 217 are also very close to the hub 222 carryingthe movable vanes 223 and 224 so that the small gaps therebetweenfunction as seals because the leakage is very small in comparison to theair which is transported through the ports 211 and 212 and the ports 213and 214.

It should be appreciated that if it is desired to provide a still betterseal, labyrinth-type seals can be utilized between the stationary vanes216, 217 and the hub 222. A flexible material such as fabric or rubberconnecting stationary vanes 216, 217 and hub 222 may also be used,although a flexible material introduces a compliance which will createor modify the low-frequency resonance of the rotary acoustic radiatorassembly. Distortion of reproduced sounds may also occur.

The surfaces of the stationary vanes 216, 217 can be provided with soundabsorption materials (not shown) to absorb the relatively high frequencysounds of seal leakage and port turbulence. The thickness of thismaterial may be significant relative to the port width as long as thismaterial is relieved near the ports so that air flow through the portsis not impeded. Such acoustic material also may serve as an effectiveshock absorbing crash barrier for the movable vanes should they overruntheir normal maximum excursion. Also, the surfaces of the movable vanesmay be covered or patterned with visco-elastic materials (not shown) fordamping of high frequency (several hundred Hertz) natural resonances inthe vanes, from the sounds originating in the bearings, and to provideminor amounts of sound absorption for seal and port air turbulencenoises.

The bearings as hereinbefore described should be as quiet as possiblebecause the cylindrical means and the stationary and movable vanes serveas efficient low-dissipation sound radiators. The loads on the bearingsare small because the motor armature, motor shaft, and rotor assemblyare statically and dynamically balanced unless there is asymmetricdynamic loading due to port obstruction.

In operation the foil strips 292 and 293 roll smoothly on and off thecapstan 291 as the shaft 219 rotates. They make no acoustic noiseperceptible with the rotary acoustic radiator 201 operating and noelectrical noise at all. The torques of the two lead assembliescounteract each other, placing substantially no net positioning torqueon the shaft 219. With this construction there are no commutatingbrushes or slip rings to wear and create noise and distortion. Ifgreater rotational excursion is required for a very high volumetricefficiency rotary acoustic radiator assembly, the two foil andtensioning leaf spring assemblies can be displaced from the same radialplane (as shown in FIG. 9) by moving one assembly longitudinallyrelative to the rotational axis of the armature 256 and securing thecapstan ends of the foils 292 and 293 so they lie beside each other forsome angular distance about the circumference of the capstan 291.

The torque motor 226 has a ratio of full torque to worst-case breakawaytorque from all sources, including bearing grease seals, of about 1000:1or 60 db. This compares very favorably with about 40:1 (32 db) forlow-inertia brush-commutated high-quality basket-wound motors. Thedifference reflects the two motors' respective propensity to distort lowlevel audio output of the rotary acoustic transducer assembly.

In FIG. 11 there is shown a diffuser-attenuator 301 suitable for therotary acoustic radiator assembly 116 of FIG. 7 and rotary acousticradiator assembly 131 of FIG. 8. Assuming that the radiator 116 in FIG.7 is used, the radial sector user-side end ports 126 and 127 are notused but user-side ports 121 and 122 in the cylinder sidewall 52 areused. The cabinet 302 is like cabinet 26 shown in FIG. 1. Cabinet ports(not shown) register with the bottom end wall ports 123 and 124 in therotary acoustic radiator assembly 116. In FIG. 12 the rotary acousticradiator assembly 305 is mounted external to the cabinet 302 on top ofthe cabinet top wall 304 while the torque motor 306 and the positionencoder 307 are mounted within the cabinet 302. The top baffle 311 ismounted above the top of the cabinet 302 by bolts and spacers (notshown). An intermediate baffle 312 is mounted over the rotary acousticradiator assembly 305 and is secured to the cabinet 302 by bolts andspacers (not shown) and roughly divides the user-side ports 308 and 309horizontally in half. Baffles 311 and 312 may be mounted to the rotaryacoustic radiator-assembly 305 to improve sinking of torque motor 307heat. Diffuser-attenuator air duct wall components 316, 317, 318 and 319are provided and are bonded between the surfaces of the top of thecabinet 302, intermediate baffle 312, and top baffle 311.

The components 316-319 are provided with outer surfaces which arenominally in alignment with the outer margins of the baffles 311 and 312as well as of the cabinet 302 (see FIG. 12). The components 316-319 areprovided with arcuate surfaces 321 which are opposite the ports 308 and309 and are spaced therefrom. Arcuate blanket components 326, 327, 328and 329 are bonded to the outside surface of the rotary acousticalradiator assembly 305 between the horizontal margins of ports 308 and309 and the top of the cabinet 306, intermediate baffle 312 and the topbaffle 311. Together these diffuser-attenuator components effectivelycreate eight air ducts 331 with throats at the user-side ports 308 and309 and mouths at the edges of the top of the cabinet 302, intermediatebaffle 312 and top baffle 311. It should be appreciated that additionalintermediate baffles and corresponding air duct wall components may beused if desired. For a small rotary acoustic radiator assembly, thediffuser-attenuator 301 may be built without an intermediate baffle. Ifuser-side end ports are included in the rotary acoustic radiatorassembly, additional components can be provided for thediffuser-attenuator, like those in FIG. 1. The diffuser-attenuator meansmay include screens or filters (not shown) to shield the parts of therotary acoustic radiator assembly from foreign bodies and also toprotect users.

FIG. 13 shows selected electromagnetically active components of anaxial-gap brushless commutated torque motor 350 having two statorassemblies 352 and a multipole rotor assembly 354. The position sensingdevice controlling commutation is the position encoder 24 describedhereinabove. The multipole rotor assembly 354 is affixed to a shaft 356supported in bearings (not shown) for rotation about a motorlongitudinal axis 358. Each stator assembly 352 is a substantially solidferromagnetic ring supported coaxially with the shaft 356 and multipolerotor assembly 354, said ferromagnetic ring having radial thickness,axial length, an inner and an outer circumference, a first axial face360 disposed proximally to the multipole rotor assembly 354 and a secondaxial face 362 disposed distally from the multipole rotor assembly 354.As shown in the example of FIG. 13 the stator assembly 352 is acomposite structure comprised of two ferromagnetic components--a woundcylindrical core 364 of ferromagnetic strip material and a stator poleassembly 366 molded of ferromagnetic powder and bonded to the woundcylindrical core 364 with structural adhesives.

A plurality of radially-oriented poles 368 and stator winding slots 370is disposed alternately at equal intervals about the first axial face360 of the stator assembly 352. Each stator pole 368 extends axiallyfrom a stator pole root 372 adjacent to the closed ends of the twoadjacent winding slots 370 to a stator pole 368 tip or face 374 disposedgenerally normal to the motor longitudinal axis 358 adjacent to themultipole rotor assembly 354. Each stator pole 368 extends radially fromthe inner circumference to the outer circumference of the statorassembly 352. A plurality of electrical conductors is placed in thestator winding slots 370 as described hereinbelow (FIG. 14).

The multipole rotor assembly 354 is comprised of a plurality of arcuatecircumferentially spaced-apart ferromagnetic rotor pole sectors 375having radial extent, angular extent, and axial thickness, each rotorpole sector 375 disposed normally to the motor longitudinal axis 358.Each rotor pole sector 375 is comprised of three layers: two pole pieces376 of generally plane ferromagnetic material having radial extent,angular extent, and axial thickness separated axially by a magnet 378,said magnet 378 being an arcuate sector of permanently magnetizedferromagnetic material having radial extent, angular extent, and axialthickness disposed normally to the motor longitudinal axis andmagnetized parallel to the motor longitudinal axis. The pole pieces 376are bonded to the magnets 378 with structural adhesives. Each magnet isso magnetized that the polarity of the magnets 378 and their axiallyadjacent pole pieces 376 alternates from rotor pole sector 375 to rotorpole sector 375 around the multipole rotor assembly 354.

Each rotor pole sector 375 is joined to a hub 380.

As shown in this FIG. 13 said hub 380 consists of two substantiallyidentical hub-half structures 382. Two such hub-half structures 382 aremounted facing each other as mirror images on the shaft 356. Eachhub-half structure 382 is comprised of a plurality of hub radial sectors384 disposed at angles about the motor longitudinal axis 358 and eachhub radial sector 384 is comprised of a plurality of radial spokes 386joined to a common central bushing 388 and to the pole pieces 376. Thetwo hub-half structures 382 are rotationally aligned on the shaft 356 sothat their spokes 386 and the pole sectors 375 affixed thereto are inangular registration about the motor longitudinal axis. The centralbushing 388 of each hub-half 382 is affixed to the shaft 356 so thatsaid rotor pole sectors 375, said hub 380 and said shaft 356 rotate as aunit. In this example it may be seen that the spokes 386 of the hub 380are disposed at angles to a plane normal to the motor longitudinal axis358 to resist the axial magnetic attraction forces between multipolerotor assembly 354 and adjacent stators assemblies 352, particularlyduring motor 350 fabrication, when said axial forces may be unequal andlarge.

When assembled in a brushless commutated torque motor 350, the multipolerotor assembly 354 is approximated to the two stator assemblies 352 witha narrow (0.010 in.) axial airgap (not shown) on each side of the rotorpole 375 sectors.

Magnetic flux from a pole piece 376 of the multipole rotor assembly 354having a given magnetic polarity enters the face 374 of a given statorpole 368, leaves the root 372 of said stator pole 368, enters the woundcylindrical core 364, traverses an arc in the wound cylindrical core 364of average length 360/(number of rotor pole sectors 375), enters theroot 372 of another stator pole 368 and leaves the face 374 of saidstator pole 368 to return to a rotor pole sector 375 of oppositemagnetic polarity. Torque is produced by currents in conductors placedin the stator slots 370 interacting with the magnetic fields of therotor pole sectors 375.

The large number of stator poles 368 limits the influence on motortorque of a single stator pole 368, i.e., if there are 12 stator poles368 for each rotor pole sector 375, the influence of one stator pole 368on total torque of that rotor pole sector 375 is roughly 8 percent, andif the stator poles 368 and their adjacent windings are alike within 10percent and do not of there own geometry generate torque ripple, anoverall rotor pole sector 375 and hence motor torque ripple of 1 percentis possible.

In this invention the rotor pole sectors 375 in the multipole rotorassembly 354 each subtends an arc substantially less than 360degrees/(number of rotor pole sectors 375), and many (4 to 32) statorpoles 368 are provided for each rotor pole sector 375. Since the statorpoles 368 are disposed at equal intervals around the motor longitudinalaxis 358, at any point in rotation of the multipole rotor assembly 354several stator poles 368 lying between rotor pole sectors 375 arewithout axially adjacent rotor pole sectors 375.

This relationship is shown in FIG. 13, where dashed radial constructionlines 390, 391, and 392 extend from the motor longitudinal axis 358along the margins of certain of the rotor pole sectors 375. Dashed lines390 and 392 extend along the clockwise edges of two adjacent rotor polesectors 375, and are separated by an angle .O slashed., which is ofextent equal to 360 degrees/(number of rotor pole sectors 375).

A third dashed radial line 391 extends along the counterclockwise edgeof rotor pole sector 375 having construction line 390 at its clockwisemargin. The angular extent of the included rotor pole sector 375 ismeasured from 390 to 391 and is .O slashed._(r). It is apparent that .Oslashed._(r) is less than .O slashed._(t), said difference being .Oslashed._(g). Dashed vertical construction lines are projected fromcorners of the rotor pole sectors 375 bounded by radial constructionlines 390, 391, and 392, indicating the axial projection of rotor polesectors 375 on the stator pole assembly 366. It is seen that betweenadjacent rotor pole sectors 375 is a gap of "uncovered" stator poles 368and the extent of this gap is .O slashed._(g).

Because flux transfers between rotor pole sectors 375 during commutationand flux transfers may not be instantaneous or simultaneous, influencedby stator winding inductance and driving circuit impedance interactionsand ferromagnetic hysteresis effects in stator assembly 352 andmultipole rotor assembly 354, commutation is usually the leastpredictable source of torque ripple. In the configuration of thisinvention commutation is caused to occur in windings between statorpoles 368 which lie in the gap .O slashed._(g) between rotor polesectors 375 and thus not axially adjacent to rotor pole sectors 375.Thus the commutating flux that affects the rotor pole sectors 375 isweak fringing flux from relatively distant stator poles 368 and statorwindings, not the intense flux in the low-reluctance narrow airgapswherein the rotor pole sectors 375 axially overlie stator poles 368, andthe influence of commutating flux changes on total multipole rotorassembly 354 torque is thereby substantially reduced.

An attractive attribute of the composite stator assembly 352 comprisedof powdered ferromagnetic pole assembly 366, having a workingpermeability of about 500, and a wound cylindrical core 364 offerromagnetic strip having a working permeability of 50,000 or more, isthat almost all of the flux from the stator pole roots 372 penetratesthe high-permeability cylindrical ferromagnetic strip core 364 axiallyand has little inclination to traverse circumferentially in thelower-permeability ferromagnetic powder pole assembly 366 to adjacentstator poles 368, limiting the influence of stator poles 368 which arecommutating on adjacent stator poles 368 which are not. Magneticallyisotropic materials such as metallic glasses are generally superior tocommon silicon steel strip as wound cylindrical cores 364, as siliconsteel strip usually has much higher longitudinal than transversepermeability and encourages circumferential flux movement in theferromagnetic powder pole assembly 366, circumferentially extending themagnetic influence of a commutating stator pole 368 to adjacent statorpoles 368. Metallic glass also possesses greater pulse permeability thandoes silicon steel, facilitating generation of short flux pulses tomanage bearing breakaway friction as described hereinabove.

The disposition of multipole rotor assembly 354 and stator poles 368shown has further advantages in limiting the influence of commutation ontorque ripple with the small oscillating angular excursionscharacteristic of a rotary acoustic transducer assembly 21, as describedhereinbelow.

FIG. 14 is an example of a stator winding pattern 400 of this invention,presented in a traditional highly schematized way, and its drivingelectronics. A multipole rotor assembly 354 is surrounded by a planerepresentation of a stator pole assembly 366 (FIG. 13) having 48 statorpole faces 372 between which there are stator winding slots 370. Asshown here, there are two groups of windings--the main windings 401which ultimately, with the multipole rotor assembly 354 components,generate almost all of the motor torque, and the pulse windings 402,used for bearing breakaway friction management, which will be describedhereinbelow. Both winding sets, main and pulse, share a common ground404.

Individual winding conductors 406 of the main drive winding set 401have, in this example, a wave winding pattern. Each of the 12 mainwinding conductors 406 is terminated in a terminal 410 and the commonground line 404. Each individual winding conductor 406 leaves a terminal410 and traverses the stator pole assembly 366 winding slots 370 in sucha way that the individual winding conductor 406 passes between thestator poles 368 in alternating radial directions, i.e., innercircumference to outer circumference followed by outer circumference toinner, and so forth until the individual winding conductor 406 returnsto the common ground 404. For example, specific individual windingconductor 412 leaves terminal 420 and follows the path 421, 422, . . .428 in numerical sequence, returning to the common ground 404. Thoughrepresented here as single turn for simplicity, in practice each of themain windings 401 will usually be comprised of many turns through thestator slots 370.

Each of the plurality of main drive windings 401 described hereinaboveis driven by an individual power amplifier 429 controlled by electronicscomprised of a position sensor 24 (not shown), a microcomputer 430, aD/A converter 432, and a microcomputer 430-controlled multiplexer 434which distributes driving signals to individual power amplifiers 429 andthence to main windings 401. Torque control and rotor assembly 354position information enter the microcomputer 430 on lines 435 and 436,respectively. Digital signals for individual stator main windings 401pass through control line 437 to be converted to analog information online 438. Main stator winding 401 selection addresses are on lines 439.Appropriate power amplifiers are economically available in groups offour amplifiers in a single integrated circuit, intended for use inautomobile stereo radio receivers.

The multiturn main windings 401 have significant inductance and aredifficult to drive with the brief pulses necessary to overcome breakawayfriction as described hereinabove. In a d'Arsonval galvanometernoncommutating torque motor as described hereinabove (FIG. 9) or themultipole brushless electric motor 350 configuration describedimmediately above (FIG. 13), the bearing breakaway friction is roughly60 db. below maximum motor torque. This low-level torque may bedelivered by pulse windings 402 having single conductor turns andsharing the same rotor assembly 354 and stator pole assembly 366 withthe main drive windings 401 as shown in FIG. 14. A typical pulse winding402 path is 441, 442, . . . 448, in numerical sequence, in all traversesof the stator pole assembly 366 the pulse winding 402 overlapping a mainwinding 401 in the same stator slot 366, as shown at 442, 444, 446, and448. The pulse windings 402 may be driven by low-power circuitry (notshown) having greater speed than is economically attractive in mainwinding individual power amplifiers 429.

In the pulse winding 402 example shown, with a multipole rotor assembly354 as shown in this FIG. 14, only two pulse windings 402 are necessary,as the two pulse windings 402 may be so disposed in the stator poleassembly 366 that for the ordinary angular rotation of a two-moving-vanerotary acoustic radiator 22, which is of the order of 100 to 130degrees, one of the pulse windings 402 will always lie under a rotorpole sector 375 and thus be capable of generating multipole rotorassembly 354 torque. Such a disposition is indicated by the short radiallines 451,452 on the rotor pole sectors 375 in FIG. 14, indicatingangular disposition of pulse windings 402 in stator slots 442 and 458when the multipole rotor assembly 354 is centered. It may be seen thatif the multipole rotor assembly 354 rotates up to 60 degreescounterclockwise (each rotor pole sector 375 of this example subtends67.5 degrees), the pulse winding at disposed in the stator slot 370 at442 will remain subtended by the rotor pole sector 375 bearing line 451.

Counterclockwise rotation will maintain an equivalent relationshipbetween pulse winding at 458 and its subtending rotor pole sector 375bearing line 452. Commutation between these two pulse windings 402 isstraightforward, using only a single bit of rotational positioninformation which may be derived from the angle registers in themicrocomputer 81 (FIG. 2a).

A further simplification can reduce the pulse windings from two to asingle winding centered under the rotor pole sectors 375 at rest anddriven only if the resting rotor pole sector 375 subtends the pulsewinding, i.e., for the rotor pole sector 375 to "uncover" the pulsewinding requires multipole rotor assembly 354 angular excursions ofroughly ±30 degrees, within 6 db. of maximum output (±60 degrees). Atthis multipole rotor assembly 354 excursion level bearing staticfrictional effects are therefore 54 db. down (6db-60db), unlikely to benoticed if the pulse windings are not energized at all.

In a rotary acoustic transducer assembly 21 the amplitude of rotationdecreases with frequency for a given perceived sound level, thelowest-audible-frequency sounds are not commonly present, and mostlistening is done at levels 10 to 30 db below peak. The consequence ofthese factors is that most multipole rotor assembly 354 excursions areof the order of 10 percent or less of maximum. The rotor pole sector 375and stator assembly 352 configuration of FIG. 13 has considerablepotential for reduction of torque ripple and attendant sound distortionif hysteresis is introduced into stator pole commutation.

FIG. 15a is a schematic representation of traditional electroniccommutation. The stator main winding 401 current I, varying from -I to+I, is shown along the ordinate and the abscissa is multipole rotorassembly 354 angular position .O slashed.. As .O slashed. increases, atsome position .O slashed._(c) the polarity of stator main windingcurrent I reverses from -I to +I. The current I path as f(.O slashed.)is 461, 462, 463. When the multipole rotor assembly 354 rotationreverses the stator main winding 401 current I is reversed at the samerotational point .O slashed._(c) and the path as f(.O slashed.) is 463,462, 461. This retrace of the identical path is idealized, and, as hasbeen described hereinabove, is generally not achieved primarily becauseof stator winding inductance and driving source impedance interactionsand stator assembly hysteresis: the consequence is commutation torqueripple. Flux may be reversed gradually, over a period greater than thatof the stator main winding 401 inductance and driving amplifier 429impedance time constant, in order to minimize these effects, assuggested by the bidirectional dashed line 464, but the effect may be tocreate an effective gap in motor torque as commutating windings aremidway through the current transition.

The current paths for changes in the two transitions -I to +I and +I to-I may be rotationally separated as shown in FIG. 15b. Here the -I to +Itransition is at rotational position .O slashed._(c2), following path471, 472, 474 as f(.O slashed.) and the +I to -I transition is at .Oslashed._(c1) following path 474, 473, 471 as f(.O slashed.). Theconsequence of this commutation hysteresis between .O slashed.c1 and .Oslashed.c2 is that the multipole rotor assembly 354 may rotate between.O slashed._(c1) and .O slashed._(c2) without commutating at all, thusintroducing no commutation torque ripple to the multipole rotor assembly354, and no distortion from this source. Gradual commutation assuggested by paths 475 and 476, may be used to reduce inductive effectsin the overall stator magnetic field as commutation occurs where rotorpole sectors 375 do not subtend the commutating stator poles 368, thoughthe commutation-free region is reduced to the angular sector between .Oslashed._(c3) and .O slashed._(c4).

In a typical application to a rotary acoustic transducer assembly 21 ahysteresis angle .O slashed._(c2) -.O slashed._(c1) might be 7.5degrees, equal to the width of a stator pole 368 and adjacent statorslot 370 in the 48-pole example as described in FIGS. 13 and 14. Thiswould mean that at the lowest frequency of 20 Hz, where the maximummultipole rotor assembly 354 excursion is perhaps 120 degrees,commutation would become limited starting at -24 db (16:1 down) frompeak output; at 80 Hz, where multipole rotor assembly 354 excursion isreduced by 4:1, the transition would start to occur at -12 db. (only 4:1down) from peak output. With sound output levels 6 db. further down,commutation ceases altogether. At listening sound levels 10-30 db. belowpeak, which is where a high-quality brush-commutated motor producesdistortion of unacceptable levels (10 percent or more) with furtherrelative distortion increases with decreasing sound level, the brushlesscommutated motor of this invention is transitioning from a commutatingregion having distortion in the 1 percent region to a non-commutatingstate akin to that of the d'Arsondescribed galvanometer torque motordrive described hereinabove. There is no relative increase ofcommutation-induced distortion below -30 db. as there is no commutationin this region.

Torque ripple from sources other than commutation, such as uneven statorpole assembly flux distribution, persists when the motor is notcommutating, but may be reduced in traditional ways, primarily, in anaxial-gap brushless commutated torque motor 350 such as that of FIG. 13,by increasing the number of stator poles 368 and by skewing rotor polesector 375 circumferential margins and stator poles 368 from theirradial alignment in the plane normal to the motor longitudinal axis 358.Additional ripple-reduction techniques may include using rotor polesectors 375 having differing angular extents and use of unequal angulardisposition of diametrically-opposed rotor pole sector 375 pairs aboutthe motor longitudinal axis 358, e.g., offsetting one diametricallyopposed rotor pole sector 375 pair by half the combined width of astator pole 368 and stator winding slot 370.

The axial-gap brushless commutated torque motor 350 described herein isa satisfactory alternative to a galvanometer-type torque motor drive foruse in a rotary acoustic transducer assembly 21. Torque linearizationtechniques described hereinabove for the galvanometer-type torque motorare applicable and reduce distortion levels below the intrinsic level ofthe brushless commutated torque motor 350 and increase its linear torquerange, as ferromagentic saturation effects can be partially compensatedby changes in main stator winding 401 drive. These torque linearizationtechniques will compensate for the peculiarities of each individualmotor, since each rotational position .O slashed. within the operationalrange is individually mapped following motor assembly and periodicallythereafter. Compensated motor peculiarities may include such mischancesas a chip on a single stator pole 368 acquired during operational use.Provision of breakaway-friction flux pulses is simplified by the use ofseparate stator pulse windings 402.

In general, the apparatus of the present invention can be generallycharacterized as a rigidly structured microcomputer-controlledtransducer comprised of rigid movable vanes rotating in a rigid portedcylinder having a variety of vane and port configurations which iscapable of reproducing sound with low distortion from a high frequencycrossover point of 80 Hz down to well below the 20 Hz lower limit of theaudio frequency spectrum. Even in a small cabinet, the frequencyresponse in and immediately below its working bandwidth is substantiallyflat and without resonances without use of negative feedback.

While this invention has been described in conjunction with a generallycylindrical chamber containing the rotatable shaft with movable vanesmounted thereto, naturally, the chamber could be of any otherappropriate shape. For example a spherical chamber would be appropriateunder certain circumstances or a toroidal chamber or portions thereof.For example the spherical chamber could be sliced such that the two endsof the sphere were removed to provide flat parallel surfaces leaving acenter spherical portion with flat ends.

Two representations of alternate ported chamber shapes are shown inFIGS. 16 and 17, having as chambers respectively a toroid of circularradial cross-section and a truncated spherical segment with parallelplane axial ends.

FIG. 16 illustrates in an exploded view a toroidal chamber incorporatingthe principals of this invention.

In FIG. 16, a toroidal chamber 551 includes an upper half 551a and alower half 551b. Within chamber 551 are mounted movable vanes 571 and572 connected by means of disk 559 to shaft 561. Shaft 561 is connectedto a motor (not shown) which causes shaft 561 to rotate in anoscillatory manner thereby to cause vanes 571 and 572 to first move in aclockwise direction and then in a counter clockwise direction aboutshaft 561. Openings 567 and 568 in bottom half of toroid 551b allow airto be either moved out of the toroidal chamber 551 or drawn into thetoroidal chamber 551 in response to moveable vanes 571 or 572 movingtoward or away from the corresponding opening. Openings 566 and 564 inthe top half 551a of toroid chamber 551 serve these same functions.Stationary vane 562 is shown mounted to the interior surface of the tophalf of 551a of toroidal chamber 551 just adjacent to port 566. Thus,when moveable vane 572 approaches stationary vane 562, air is forced outof the toroidal chamber 551 through port 566. Simultaneously, as movablevane 571 approaches port 564, a stationary vane 563 (not shown) mountedadjacent to port 564 diametrically opposite to stationary vane 562forces the air being displaced by moveable vane 571 as it moves towardport 564 out through port 564. While vane 571 is moving toward port 564,air is drawn in through port 567 in the bottom half of 551b of toroidalchamber 551 and simultaneously as movable vane 572 moves toward port566, air is drawn into the toroidal chamber through port 568 in bottomhalf 551b of toroidal chamber 551. Stationary vanes 562 and 563 (notshown) are also connected to the interior surface of the bottom half551b of the toroidal chamber 551 in such a manner as to force air beingdisplaced by movable vane 572 as moveable vane 572 moves towardstationary vane 562 out of chamber 551 through port 566 and not throughport 567.

FIG. 17 illustrates in exploded view spherical chamber 661 comprised ofa sphere, the top and bottom sections of which have been removed to forma plane upper surface 662a and a plane lower surface 662b parallel tothe upper surface 662a. Surfaces 662a and 662b are both perpendicular tothe axis of rotatable shaft 661. Movable vanes 671 and 672 are mountedon rotatable shaft 661 by means of hub 659 which is affixed to shaft661. When shaft 661 is rotated in an oscillatory manner by a motor (notshown) movable vanes 671 and 672 rotate first in one direction and thenin the opposite direction. Stationary vane 662 and a diametricallyopposite corresponding stationary vane 663 (not shown) are affixed tothe interior surface of chamber 661. Stationary vanes 662 and 663 forceair out of the interior chamber 661 or allow air to be drawn intochamber 661 through ports 664, 666, 668 and 670 (not shown) dependingupon the direction of rotation of shaft 661 and movable vanes 671 and672. Thus, as movable vane 671 is rotated toward stationary vane 662(which is attached to the interior surface of both lower half 661b ofthe spheroidal chamber and upper half 661a of the spherical chamber),air is forced out of chamber 661 through port 664. Air is also forcedout of chamber 661 through port 666 by movable vane 672. When shaft 661is rotating in the opposite direction, air is drawn into chamber 661through ports 664 and 666 and forced out of chamber 661 through port 668and a diametrically opposite port (670 not shown) on the side of thebottom portion 662b of chamber 661 opposite port 668.

The embodiments of FIGS. 16 and 17 illustrate two rotary acousticradiators in accordance with the principals of this invention.

While certain embodiments of this invention have been described, otherembodiments of this invention will be obvious in view of the abovedescription. Numerous changes and modifications can be made in theembodiments disclosed without departing from the invention.

What is claimed is:
 1. A rotary acoustic transducer apparatus forproducing sound in response to an applied electrical signal,comprising:a rotary acoustic radiator assembly comprising:a generallycylindrical chamber with a cylindrical sidewall and end walls having anaxis, a shaft, bearings mounting said shaft in said cylindrical meansfor rotation about said axis, a cylindrical hub secured to said shaftand extending between said end walls, movable vanes secured to saidshaft, said shaft, said hub, and said movable vanes forming a rotorassembly, stationary vanes mounted in said chamber between said movablevanes and extending between said cylindrical sidewall and said hub andbetween said end walls, said cylindrical chamber having ports openingthrough the walls of said cylindrical chamber to direct air flow intoand out of the cylinder in response to movement of the movable vanes, atorque motor coupled to the shaft for applying rotational reciprocatingmovement to the rotor assembly, a position encoder for ascertaining theposition of said rotor assembly, and a microcomputer operatively coupledto said torque motor and to said position encoder for generating a drivesignal to drive said torque motor from said applied electrical signaland from corrections to said applied electrical signal derived by saidmicrocomputer.
 2. Apparatus as in claim 1 includinga cabinet in the userenvironment and means for mounting said rotary acoustic radiatorassembly in operative relation to said cabinet so that certain of saidports open into the cabinet and certain of said ports open externally ofthe cabinet and produce an air flow into and out of the user environmentupon rotation of said rotor assembly.
 3. Apparatus as in claim 1includinga diffuser attenuator means disposed in the vicinity of saidrotary acoustic radiator for redirecting and slowing air flow to andfrom ports opening into the user environment.
 4. Apparatus as in claim 3wherein the diffuser attenuator means includes acoustic absorbentmaterial to reduce noise in the air flow passing through the diffuserattenuator.
 5. Apparatus as in claim 1 wherein the microcomputerincludes means for generating a centering signal in the drive signal tothe torque motor for maintaining the average position of the rotorassembly in the center of its rotational travel arc limits.
 6. Apparatusas in claim 1 wherein the microcomputer includes means for preventingcontact of the moving vanes of the rotor assembly and the stationaryvanes by appropriately modifying said drive signal.
 7. Apparatus as inclaim 1 wherein the microcomputer includes means for calibrating motortorque linearity as a function of rotational position of said rotorassembly and for operating on the applied electrical signal tocompensate for any torque motor nonlinearity.
 8. Apparatus as in claim 1wherein the microcomputer includes means for measuring air compliance asa function of the position of the rotor assembly and operating on theapplied electrical signal to compensate for any air compliancenonlinearity.
 9. Apparatus as in claim 1 wherein the microcomputerincludes means for measuring air leakage as a function of rotor assemblyposition and operating on the applied electrical signal to compensatefor the leakage.
 10. Apparatus as in claim 1 wherein the microcomputerincludes means for measuring bearing breakaway friction as a function ofboth the extent and velocity of bearing rotation prior to stop and ofthe duration of the bearing stop at local peaks and plateaus of theapplied electrical signal, and operating on the applied electricalsignal to provide said drive signal to the torque motor which producestorque when bearing rotation resumes to overcome the bearing breakawayfriction.
 11. Apparatus as in claim 1 wherein the microcomputer includesmeans for providing an infrasonic signal as part of the drive signal tothe torque motor to continue bearing rotation at peaks and plateaus insaid applied electrical signal to forestall extended bearingnon-rotation.
 12. Apparatus as in claim 1 includingmeans for delaying afew milliseconds said applied electrical signal to facilitate bearingbreakaway friction management by sampling and assessing the appliedelectrical signal prior to its being supplied to the torque motor by themicrocomputer.
 13. Apparatus as in claim 1 wherein the microcomputerincludes means for sensing torque motor armature temperature and coolingthe torque motor by providing an infrasonic signal as part of the drivesignal to the torque motor to produce infrasonic oscillation of therotor assembly.
 14. Apparatus as in claim 5 wherein the microcomputerincludes means for generating an infrasonic signal to modulate thecentering signal in the drive signal to the torque motor so that theoperating center of said torque motor and rotor assembly wanders slowlyabout an index position to distribute bearing wear.
 15. A rotaryacoustic radiator assembly for producing sound in response to areciprocating rotational movement of a torque motor driven by an appliedelectrical signal, said rotary acoustic radiator assembly comprising:agenerally cylindrical chamber with a cylindrical sidewall and end walls,said chamber having an axis; a shaft; bearings mounting said shaft insaid cylindrical chamber for rotation about said axis; a cylindrical hubsecured to said shaft and extending between said end walls; movablevanes secured to said shaft; said shaft, said hub, and said movablevanes forming a rotor assembly; stationary vanes mounted in said chamberbetween said movable vanes and extending between said cylindricalsidewall and said hub and between said end walls; said cylindricalchamber having ports opening through the end walls of said cylindricalchamber adjacent to each stationary vane, each stationary vane havingassociated therewith a port in an end wall on one side of saidstationary vane and a port in the other end wall on the other side ofsaid stationary vane to permit air flow into and out of the cylinder inresponse to movement of the movable vanes.
 16. A rotary acousticradiator assembly for producing sound in response to a reciprocatingrotational movement of a torque motor driven by an applied electricalsignal, said rotary acoustic radiator assembly comprising:a generallycylindrical chamber with a cylindrical sidewall and end walls, saidchamber having an axis; a shaft; bearings mounting said shaft in saidcylindrical chamber for rotation about said axis; a cylindrical hubsecured to said shaft and extending between said end walls; movablevanes secured to said shaft; said shaft, said hub, and said movablevanes forming a rotor assembly; stationary vanes mounted in said chamberbetween said movable vanes and extending between said cylindricalsidewall and said hub and between said end walls; said cylindricalchamber having ports opening through the end walls of said cylindricalchamber adjacent to each stationary vane, each stationary vane havingassociated therewith a port in an end wall on one side of saidstationary vane and a port in the other end wall on the other side ofsaid stationary vane to permit air flow into and out of the cylinder inresponse to movement of the movable vanes, said cylindrical chamberincluding additional ports formed in the cylindrical sidewall adjacentto each stationary vane.
 17. A rotary acoustic radiator assembly forproducing sound in response to a reciprocating rotational movement of atorque motor driven by an applied electrical signal, said rotaryacoustic radiator assembly comprising:a generally cylindrical chamberwith a cylindrical sidewall and end walls having an axis; a shaft;bearings mounting said shaft in said cylindrical means for rotationabout said axis; a cylindrical hub secured to said shaft and extendingbetween said end walls; movable vanes secured to said shaft; said shaft,said hub, and said movable vanes forming a rotor assembly; stationaryvanes mounted in said chamber between said movable vanes and extendingbetween said cylindrical sidewall and said hub and between said endwalls; said cylindrical chamber having ports opening through thecylindrical chamber adjacent to each stationary vane, each stationaryvane having associated therewith a sidewall port on one side of saidstationary vane and an end wall port on the other side of saidstationary vane to permit air flow into and out of the cylinder inresponse to movement of the movable vanes.
 18. A rotary acousticradiator assembly for producing sound in response to a reciprocatingrotational movement of a torque motor driven by an electrical signal,said rotary acoustic radiator assembly comprising:structure with acylindrical sidewall and end walls forming a cylindrical chamber havingan axis, a shaft, bearings mounting said shaft in said cylindricalchamber for rotation about said axis, a cylindrical hub secured to saidshaft and extending between said end walls, movable vanes secured tosaid hub, said shaft, said hub, and said movable vanes forming a rotorassembly, stationary vanes mounted in said chamber between said movablevanes and extending between said cylindrical sidewall and said hub andbetween said end walls, each of said movable vanes and stationary vaneshaving a finite thickness and first and second faces and a centersurface midway between said first and second faces, said center surfacehaving at least four corners, at least one of said movable andstationary vanes having at least four corners of the center surfaceabutting the end walls, two corners abutting the cylindrical hub and twocorners abutting the cylindrical sidewall, said center surface of eachof said vanes being formed such that four planes, each defined by a linewhich is the chamber axis and by a point which is one of said fourcorners of said center surface, lie in more than one plane, saidcylindrical chamber having ports opening through the walls of saidcylindrical chamber to direct air flow into and out of the cylindricalchamber in response to movement of the movable vanes.
 19. A rotaryacoustic radiator assembly for producing sound in response to areciprocating rotational movement of a torque motor driven by anelectrical signal, said rotary acoustic radiator assemblycomprising:structure with a cylindrical sidewall and end walls forming acylindrical chamber having an axis, a shaft, bearings mounting saidshaft in said cylindrical chamber for rotation about said axis, movablevanes secured to said shaft, said shaft and said movable vanes forming arotor assembly, stationary vanes mounted in said chamber between saidmovable vanes and extending between said cylindrical sidewall and saidshaft and between said end walls, said cylindrical chamber having portsopening through at least one of the end walls and the sidewall and intosaid cylindrical chamber to permit air flow into and out of saidcylindrical chamber in response to movement of the movable vanes, atleast two of said ports in the cylindrical chamber walls being at leastpartially superposed in the same angular sector about the axis.
 20. Arotary acoustic radiator assembly for producing sound in response to areciprocating rotational movement of a torque motor driven by anelectrical signal, said rotary acoustic radiator assembly comprising:astructure with a cylindrical sidewall and end walls forming acylindrical chamber having an axis, a shaft, bearings mounting saidshaft in said cylindrical means for rotation about said axis, acylindrical hub secured to said shaft and extending between said endwalls, movable vanes secured to said shaft, said shaft, said hub, andsaid movable vanes forming a rotor assembly, stationary vanes mounted insaid chamber between said movable vanes and extending between saidcylindrical sidewall and said hub and between said end walls, saidcylindrical chamber having certain ports opening through an end wall ofsaid cylindrical chamber to permit air flow into and out of thecylindrical chamber in response to movement of the movable vanes, saidcertain ports being arranged so that air flow therethrough passesthrough said torque motor for cooling the torque motor.
 21. In a rotaryacoustic transducer apparatus for producing sound in response to anapplied audio signal, comprising:a rotary acoustic radiator assembly anda torque motor, the rotary acoustic radiator assembly comprising agenerally cylindrical structure with a cylindrical sidewall and endwalls forming a cylindrical chamber having an axis, a shaft, bearingsmounting said shaft in said cylindrical means for rotation about saidaxis, a cylindrical hub secured to said shaft and extending between saidend walls, movable vanes secured to said shaft, said shaft, said hub,and said movable vanes forming a rotor assembly, stationary vanesmounted in said chamber between said movable vanes and extending betweensaid cylindrical sidewall and said hub and between said end walls, saidcylindrical means having ports opening through at least certain of thewalls to permit air flow into and out of the cylinder in response tomovement of the movable vanes, said torque motor coupled to the shaftapplying rotational reciprocating movement in accordance with saidapplied electrical signal, said torque motor including a torque motorshaft and an armature mounted on the shaft and a capstan formed of aninsulating material mounted on said torque motor shaft, first and secondconducting metal foil strips secured to said capstan at separatepositions thereon having first ends connected to said armature andhaving second ends supported and tensioned individually by conductiveleaf springs so that the capstan may rotate over a limited range whilemaintaining electrical continuity through said metal foil strips andleaf springs, and means connecting said leaf springs to the appliedelectrical signal.
 22. A rotary acoustic transducer for producing soundin response to an electrical signal in a user environment, comprising:arotary acoustic radiator assembly and torque motor, the rotary acousticradiator assembly comprising a generally cylindrical chamber with acylindrical sidewall and end walls forming a cylindrical chamber havingan axis, a shaft, bearings mounting said shaft in said cylindrical meansfor rotation about said axis, a cylindrical hub secured to said shaftand extending between said end walls, movable vanes secured to saidshaft, said shaft, said hub, and said movable vanes forming a rotorassembly, stationary vanes mounted in said chamber between said movablevanes and extending between said cylindrical sidewall and said hub andbetween said end walls, said cylindrical means having ports opening intosaid cylindrical chamber and into the user environment to permit airflow into and out of the cylindrical chamber in response to movement ofthe movable vanes, said torque motor being coupled to said shaft andapplying rotational reciprocating movement in accordance with theapplied electrical signal and diffuser attenuator means disposed in thevicinity of said acoustic radiator for redirecting and slowing air flowinto and from ports opening into the user environment.
 23. Apparatus asin claim 22 wherein the diffuser attenuator means includes acousticabsorbent material to reduce noise in the air flow passing through thediffuser attenuator.
 24. In a method for producing low frequency soundfrom an applied electrical signal by the use of a rotary acoustictransducer apparatus comprising an enclosure providing a cylindricalchamber having one set of stationary vanes mounted therein and having arotor assembly mounted therein having one set of movable vanes mountedthereon, the enclosure having ports therein in communication with thecylindrical chamber, torque motor means coupled to said rotor assemblyfor causing rotary reciprocating movement of said rotor assembly, and amicrocomputer for receiving the applied electrical signal and forsupplying a drive signal to the torque motor means, the microcomputerhaving a memory with at least one table of information having valuesstored therein related to a physical characteristic of the rotaryacoustic transducer apparatus including rotor assembly position, themethod comprising the steps of:causing the microcomputer to sense theposition of the rotor assembly and to read values from said at least onetable, and utilizing the sensed rotor assembly position and the readvalues from said at least one table to control the operation of therotor assembly.
 25. The method as in claim 24 together with the stepof:measuring physical characteristics of the rotary acoustic transducerapparatus; and creating additional tables in microcomputer memory forreference during rotary acoustic transducer apparatus operation.
 26. Themethod as in claim 25 together with the step of updating said additionaltables in microcomputer memory during operation for reference duringrotary acoustic transducer apparatus operation.
 27. The method as inclaim 26 wherein bearing means is provided for rotatably mounting therotor assembly, said bearing means having breakaway friction, togetherwith means for measuring the bearing breakaway friction as a function ofthe extent and velocity of rotation of the bearing prior to a stop,together with the steps of:causing the microcomputer to measure thebearing breakaway friction; and operating on the applied electricalsignal to provide torque when the bearing rotation is to resume after astop to overcome the breakaway friction.
 28. The method as in claim 26together with the step of causing the microcomputer to cause generationof an infrasonic signal in the drive signal to the torque motor tocontinue bearing rotation at local waveform peaks of the electricalsignal forestall extended bearing non-rotation.
 29. The method as inclaim 26 together with the step of causing the microcomputer to providea delay in the drive signal supplied to the torque motor to facilitatebearing breakaway friction management by sampling and assessing theapplied electrical signal prior to its being supplied to the torquemotor.
 30. The method as in claim 26 together with the step of:causingthe microcomputer to sense torque motor armature temperature and togenerate an infrasonic signal in the drive signal in response to themeasured armature temperature to produce infrasonic oscillation of therotor assembly for cooling the torque motor.
 31. The method as in claim25 together with the step of causing the microcomputer to calibrate thelinearity of the torque from the torque motor as a function ofrotational position of the torque motor and operate on the appliedelectrical signal to compensate for any nonlinearity in the torquemotor.
 32. The method as in claim 25 together with the step of causingthe microcomputer to measure air compliance in the chamber as a functionof the position of the rotor assembly and operate on the appliedelectrical signal to compensate for any nonlinearities in the aircompliance.
 33. The method as in claim 25 together with the step ofcausing the microcomputer to measure vane and air leakage as a functionof rotor assembly position and operate on the applied electrical signalto compensate for any measured leakage.
 34. The method as in claim 24together with the step of causing the microcomputer to generatecentering signals in the drive signal to the torque motor to maintainthe average position of the rotor assembly in the center of itsrotational arc limits.
 35. The method as in claim 34 together with thestep of causing the microcomputer to generate an infrasonic signal tomodulate the centering signals in the drive signal for the torque motorand rotor assembly so that the operating center of the rotor assemblywanders slowly to distribute bearing wear.
 36. The method as in claim 24together with the step of causing the microcomputer to prevent contactof the movable vanes of the rotor assembly and the stationary vanes byoperating on the applied electrical signal.
 37. A method for producinglow frequency sound by the use of an applied electrical signal by theuse of one set of stationary vanes and one set of movable vanes mountedin a cylindrical housing having at least one end wall having end portstherein, said method comprising:moving the movable vanes in the housingin accordance with the applied electrical signal to cause the movablevanes to move towards and away from the stationary vanes to cause airflow to pass rapidly into and out of the end ports to create lowfrequency sound.
 38. A method for producing low frequency sound by theuse of an applied electrical signal by the use of one set of stationaryvanes and one set of movable vanes mounted in a cylindrical housinghaving at least one end wall having end ports therein, said methodcomprising:moving the movable vanes in the housing in accordance withthe applied electrical signal to cause the movable vanes to move towardsand away from the stationary vanes to cause air flow to pass into andout of the end ports to create low frequency sound and slowing andredirecting the air flow into and out of certain of said ports.
 39. Amethod as in claim 38 together with the step of absorbing high frequencynoise from the air flow into and out of certain of said ports.
 40. Amethod as in claim 38 together with the step of screening the air flowinto certain of said ports.
 41. A rotary acoustic transducer apparatusfor producing sound in response to an applied electrical signal,comprising:a chamber having a wall or walls with ports opening throughsaid wall or walls to allow air to flow into and out of the chamber; ashaft, rotatably mounted on the axis of said chamber; movable vanessecured to said shaft, thereby to form with said shaft a rotor assembly,stationary vanes mounted in said chamber between said movable vanes andextending between said wall or walls a motor coupled to said shaft forapplying torque to the rotor assembly such that said torque generated bysaid motor in response to an electrical signal applies torque to therotor assembly, causing the rotor assembly to rotate and thereforecausing air to flow into and out of the chamber, producing sound. 42.Apparatus as in claim 41 wherein said chamber is spherical. 43.Apparatus as in claim 41 wherein said chamber is toroidal.
 44. Apparatusas in claim 41 wherein said chamber has a surface other than a cylinderdefined by the perimeter of an area rotated about said axis which liesin the same plane as the area.
 45. A rotary acoustic radiator assemblyfor producing sound in response to a reciprocating rotational movementof a torque motor driven by an electrical signal, said rotary acousticradiator assembly comprising:structure with a chamber mounted about alongitudinal axis, said chamber having a wall; a shaft mounted alongsaid axis so as to be capable of rotating about said axis; moveablevanes secured to said shaft; said shaft and said moveable vanes forminga rotor assembly; stationary vanes mounted in said chamber on said wallbetween said moveable vanes and extending between said wall and saidshaft, each of said moveable vanes and stationary vanes having a finitethickness and first and second faces and a virtual surface which is thelocus of all points midway between said first and second faces, saidvirtual surface having an edge, said edge having at least three pointson said edge such that at least one of said moveable and stationaryvanes has the virtual surface formed such that at least three planeseach defined by the longitudinal axis and by one of the three points onthe edge lie in more than one plane, said chamber having ports openingthrough the wall of said chamber to direct airflow into and out of thechamber in response to movement of the moveable vanes.